I've hit this multiple times during last week. The cause for me might be either graphics adapter driver (i915) or VirtualBox kernel driver (vbox*). Here's a portion of /var/log/kern.log (hostname replaced with "localhost"):

Jun 18 08:35:12 localhost kernel: [ 1697.352128] BUG: soft lockup - CPU#3 stuck for 22s! [Xorg:1317]
Jun 18 08:35:12 localhost kernel: [ 1697.352130] Modules linked in: pci_stub vboxpci(O) vboxnetadp(O) vboxnetflt(O) vboxdrv(O) bnep rfcomm bluetooth parport_pc ppdev snd_hda_codec_hdmi snd_hda_codec_realtek usbhid hid uvcvideo videodev snd_usb_audio v4l2_compat_ioctl32 snd_usbmidi_lib eeepc_wmi snd_hda_intel snd_hda_codec snd_hwdep asus_wmi snd_pcm sparse_keymap i915 snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq drm_kms_helper snd_timer drm snd_seq_device psmouse snd serio_raw soundcore snd_page_alloc i2c_algo_bit mei(C) video mac_hid wmi lp parport usb_storage r8169
Jun 18 08:35:12 localhost kernel: [ 1697.352157] CPU 3 
Jun 18 08:35:12 localhost kernel: [ 1697.352158] Modules linked in: pci_stub vboxpci(O) vboxnetadp(O) vboxnetflt(O) vboxdrv(O) bnep rfcomm bluetooth parport_pc ppdev snd_hda_codec_hdmi snd_hda_codec_realtek usbhid hid uvcvideo videodev snd_usb_audio v4l2_compat_ioctl32 snd_usbmidi_lib eeepc_wmi snd_hda_intel snd_hda_codec snd_hwdep asus_wmi snd_pcm sparse_keymap i915 snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq drm_kms_helper snd_timer drm snd_seq_device psmouse snd serio_raw soundcore snd_page_alloc i2c_algo_bit mei(C) video mac_hid wmi lp parport usb_storage r8169
Jun 18 08:35:12 localhost kernel: [ 1697.352179] 
Jun 18 08:35:12 localhost kernel: [ 1697.352180] Pid: 1317, comm: Xorg Tainted: G         C O 3.2.0-48-generic #74-Ubuntu System manufacturer System Product Name/P8H77-M PRO
Jun 18 08:35:12 localhost kernel: [ 1697.352183] RIP: 0010:[<ffffffff810a35a2>]  [<ffffffff810a35a2>] smp_call_function_many+0x212/0x280
Jun 18 08:35:12 localhost kernel: [ 1697.352189] RSP: 0000:ffff880401b1db68  EFLAGS: 00003202
Jun 18 08:35:12 localhost kernel: [ 1697.352190] RAX: 00000000000008fc RBX: ffffffff81312710 RCX: 0000000000000008
Jun 18 08:35:12 localhost kernel: [ 1697.352191] RDX: 00000000f7000000 RSI: ffffffff81806c60 RDI: 0000000000003293
Jun 18 08:35:12 localhost kernel: [ 1697.352193] RBP: ffff880401b1dbb8 R08: 0000000000000000 R09: 0000000000000004
Jun 18 08:35:12 localhost kernel: [ 1697.352194] R10: 0000000000000004 R11: 0000000000000000 R12: 0000000000003293
Jun 18 08:35:12 localhost kernel: [ 1697.352196] R13: ffff880401b1db18 R14: 0000000800000000 R15: 0000000000000000
Jun 18 08:35:12 localhost kernel: [ 1697.352197] FS:  00007f9489d1f880(0000) GS:ffff88041fac0000(0000) knlGS:0000000000000000
Jun 18 08:35:12 localhost kernel: [ 1697.352199] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
Jun 18 08:35:12 localhost kernel: [ 1697.352200] CR2: 00007f947e59f240 CR3: 00000004014f5000 CR4: 00000000001426e0
Jun 18 08:35:12 localhost kernel: [ 1697.352202] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
Jun 18 08:35:12 localhost kernel: [ 1697.352203] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Jun 18 08:35:12 localhost kernel: [ 1697.352205] Process Xorg (pid: 1317, threadinfo ffff880401b1c000, task ffff8804037cdc00)
Jun 18 08:35:12 localhost kernel: [ 1697.352206] Stack:
Jun 18 08:35:12 localhost kernel: [ 1697.352207]  0000000000100000 0100000000000000 0000000000000000 ffffffffa0135320
Jun 18 08:35:12 localhost kernel: [ 1697.352210]  000000000010ef60 ffffffffa0135320 0000000000000000 ffff8804049d06a0
Jun 18 08:35:12 localhost kernel: [ 1697.352213]  0000000000000000 ffff880401957800 ffff880401b1dbc8 ffffffff810a3632
Jun 18 08:35:12 localhost kernel: [ 1697.352215] Call Trace:
Jun 18 08:35:12 localhost kernel: [ 1697.352225]  [<ffffffffa0135320>] ? i915_restore_state+0x2a0/0x2a0 [i915]
Jun 18 08:35:12 localhost kernel: [ 1697.352230]  [<ffffffffa0135320>] ? i915_restore_state+0x2a0/0x2a0 [i915]
Jun 18 08:35:12 localhost kernel: [ 1697.352232]  [<ffffffff810a3632>] smp_call_function+0x22/0x30
Jun 18 08:35:12 localhost kernel: [ 1697.352235]  [<ffffffff810a3668>] on_each_cpu+0x28/0x60
Jun 18 08:35:12 localhost kernel: [ 1697.352241]  [<ffffffffa01388e3>] i915_gem_object_get_fence+0x2d3/0x310 [i915]
Jun 18 08:35:12 localhost kernel: [ 1697.352247]  [<ffffffffa013a41a>] i915_gem_fault+0x24a/0x2a0 [i915]
Jun 18 08:35:12 localhost kernel: [ 1697.352250]  [<ffffffff8113ba72>] __do_fault+0x72/0x550
Jun 18 08:35:12 localhost kernel: [ 1697.352253]  [<ffffffff8152ecdc>] ? do_sock_read.isra.12+0xdc/0xf0
Jun 18 08:35:12 localhost kernel: [ 1697.352255]  [<ffffffff8113f3aa>] handle_pte_fault+0xfa/0x200
Jun 18 08:35:12 localhost kernel: [ 1697.352261]  [<ffffffffa0139691>] ? i915_gem_set_domain_ioctl+0xd1/0x110 [i915]
Jun 18 08:35:12 localhost kernel: [ 1697.352263]  [<ffffffff81140689>] handle_mm_fault+0x269/0x370
Jun 18 08:35:12 localhost kernel: [ 1697.352266]  [<ffffffff8152ed9d>] ? sock_aio_read+0x2d/0x40
Jun 18 08:35:12 localhost kernel: [ 1697.352269]  [<ffffffff81663bbe>] do_page_fault+0x17e/0x540
Jun 18 08:35:12 localhost kernel: [ 1697.352272]  [<ffffffff8118c30a>] ? do_vfs_ioctl+0x8a/0x340
Jun 18 08:35:12 localhost kernel: [ 1697.352274]  [<ffffffff8117a30d>] ? vfs_read+0x10d/0x180
Jun 18 08:35:12 localhost kernel: [ 1697.352277]  [<ffffffff816607f5>] page_fault+0x25/0x30
Jun 18 08:35:12 localhost kernel: [ 1697.352278] Code: e8 34 cb 5b 00 0f ae f0 4c 89 e7 ff 15 40 2e b7 00 80 7d bf 00 0f 84 77 fe ff ff f6 43 20 01 0f 84 6d fe ff ff 0f 1f 40 00 f3 90 <f6> 43 20 01 75 f8 e9 5c fe ff ff 0f 1f 00 4c 89 ea 4c 89 fe 44 
Jun 18 08:35:12 localhost kernel: [ 1697.352304] Call Trace:
Jun 18 08:35:12 localhost kernel: [ 1697.352309]  [<ffffffffa0135320>] ? i915_restore_state+0x2a0/0x2a0 [i915]
Jun 18 08:35:12 localhost kernel: [ 1697.352314]  [<ffffffffa0135320>] ? i915_restore_state+0x2a0/0x2a0 [i915]
Jun 18 08:35:12 localhost kernel: [ 1697.352316]  [<ffffffff810a3632>] smp_call_function+0x22/0x30
Jun 18 08:35:12 localhost kernel: [ 1697.352318]  [<ffffffff810a3668>] on_each_cpu+0x28/0x60
Jun 18 08:35:12 localhost kernel: [ 1697.352324]  [<ffffffffa01388e3>] i915_gem_object_get_fence+0x2d3/0x310 [i915]
Jun 18 08:35:12 localhost kernel: [ 1697.352329]  [<ffffffffa013a41a>] i915_gem_fault+0x24a/0x2a0 [i915]
Jun 18 08:35:12 localhost kernel: [ 1697.352331]  [<ffffffff8113ba72>] __do_fault+0x72/0x550
Jun 18 08:35:12 localhost kernel: [ 1697.352334]  [<ffffffff8152ecdc>] ? do_sock_read.isra.12+0xdc/0xf0
Jun 18 08:35:12 localhost kernel: [ 1697.352336]  [<ffffffff8113f3aa>] handle_pte_fault+0xfa/0x200
Jun 18 08:35:12 localhost kernel: [ 1697.352341]  [<ffffffffa0139691>] ? i915_gem_set_domain_ioctl+0xd1/0x110 [i915]
Jun 18 08:35:12 localhost kernel: [ 1697.352343]  [<ffffffff81140689>] handle_mm_fault+0x269/0x370
Jun 18 08:35:12 localhost kernel: [ 1697.352346]  [<ffffffff8152ed9d>] ? sock_aio_read+0x2d/0x40
Jun 18 08:35:12 localhost kernel: [ 1697.352348]  [<ffffffff81663bbe>] do_page_fault+0x17e/0x540
Jun 18 08:35:12 localhost kernel: [ 1697.352351]  [<ffffffff8118c30a>] ? do_vfs_ioctl+0x8a/0x340
Jun 18 08:35:12 localhost kernel: [ 1697.352353]  [<ffffffff8117a30d>] ? vfs_read+0x10d/0x180
Jun 18 08:35:12 localhost kernel: [ 1697.352355]  [<ffffffff816607f5>] page_fault+0x25/0x30


Jun 18 08:35:12 localhost kernel: [ 1697.719429] BUG: soft lockup - CPU#7 stuck for 23s! [EMT-0:5538]
Jun 18 08:35:12 localhost kernel: [ 1697.719430] Modules linked in: pci_stub vboxpci(O) vboxnetadp(O) vboxnetflt(O) vboxdrv(O) bnep rfcomm bluetooth parport_pc ppdev snd_hda_codec_hdmi snd_hda_codec_realtek usbhid hid uvcvideo videodev snd_usb_audio v4l2_compat_ioctl32 snd_usbmidi_lib eeepc_wmi snd_hda_intel snd_hda_codec snd_hwdep asus_wmi snd_pcm sparse_keymap i915 snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq drm_kms_helper snd_timer drm snd_seq_device psmouse snd serio_raw soundcore snd_page_alloc i2c_algo_bit mei(C) video mac_hid wmi lp parport usb_storage r8169
Jun 18 08:35:12 localhost kernel: [ 1697.719454] CPU 7 
Jun 18 08:35:12 localhost kernel: [ 1697.719455] Modules linked in: pci_stub vboxpci(O) vboxnetadp(O) vboxnetflt(O) vboxdrv(O) bnep rfcomm bluetooth parport_pc ppdev snd_hda_codec_hdmi snd_hda_codec_realtek usbhid hid uvcvideo videodev snd_usb_audio v4l2_compat_ioctl32 snd_usbmidi_lib eeepc_wmi snd_hda_intel snd_hda_codec snd_hwdep asus_wmi snd_pcm sparse_keymap i915 snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq drm_kms_helper snd_timer drm snd_seq_device psmouse snd serio_raw soundcore snd_page_alloc i2c_algo_bit mei(C) video mac_hid wmi lp parport usb_storage r8169
Jun 18 08:35:12 localhost kernel: [ 1697.719475] 
Jun 18 08:35:12 localhost kernel: [ 1697.719477] Pid: 5538, comm: EMT-0 Tainted: G         C O 3.2.0-48-generic #74-Ubuntu System manufacturer System Product Name/P8H77-M PRO
Jun 18 08:35:12 localhost kernel: [ 1697.719480] RIP: 0010:[<ffffffff810a31de>]  [<ffffffff810a31de>] smp_call_function_single+0xae/0x160
Jun 18 08:35:12 localhost kernel: [ 1697.719484] RSP: 0018:ffff880137fc5a38  EFLAGS: 00000202
Jun 18 08:35:12 localhost kernel: [ 1697.719486] RAX: 00000000fffffffa RBX: 0000000000000246 RCX: 0000000000000000
Jun 18 08:35:12 localhost kernel: [ 1697.719487] RDX: 0000000000000000 RSI: ffff88041fbd4340 RDI: 0000000000000000
Jun 18 08:35:12 localhost kernel: [ 1697.719489] RBP: ffff880137fc5a98 R08: 0000000000000000 R09: 00000000000000fd
Jun 18 08:35:12 localhost kernel: [ 1697.719490] R10: ffff880137fc5b98 R11: 00000000ffffffff R12: ffffc900180271b0
Jun 18 08:35:12 localhost kernel: [ 1697.719492] R13: ffff880137fc59c8 R14: ffffffffa04164cb R15: ffff880137fc59a8
Jun 18 08:35:12 localhost kernel: [ 1697.719494] FS:  00007fa9e4fcf700(0000) GS:ffff88041fbc0000(0000) knlGS:0000000000000000
Jun 18 08:35:12 localhost kernel: [ 1697.719495] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
Jun 18 08:35:12 localhost kernel: [ 1697.719497] CR2: 00000000b7727ec0 CR3: 0000000135a7d000 CR4: 00000000001426e0
Jun 18 08:35:12 localhost kernel: [ 1697.719498] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
Jun 18 08:35:12 localhost kernel: [ 1697.719500] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Jun 18 08:35:12 localhost kernel: [ 1697.719501] Process EMT-0 (pid: 5538, threadinfo ffff880137fc4000, task ffff880403a52e00)
Jun 18 08:35:12 localhost kernel: [ 1697.719503] Stack:
Jun 18 08:35:12 localhost kernel: [ 1697.719504]  ffffc90018026000 0000000000000000 0000000000000000 0000000000000000
Jun 18 08:35:12 localhost kernel: [ 1697.719506]  0000000000000000 0000000000000000 0000000000000000 0000000000000000
Jun 18 08:35:12 localhost kernel: [ 1697.719509]  0000000000000000 ffffc90018026000 0000000000000001 00000000fffffc06
Jun 18 08:35:12 localhost kernel: [ 1697.719512] Call Trace:
Jun 18 08:35:12 localhost kernel: [ 1697.719522]  [<ffffffffa03845ab>] VBoxHost_RTMpPokeCpu+0x4b/0x60 [vboxdrv]
Jun 18 08:35:12 localhost kernel: [ 1697.719528]  [<ffffffff810a0516>] ? futex_wake_op+0x3b6/0x520
Jun 18 08:35:12 localhost kernel: [ 1697.719534]  [<ffffffffa0377bcf>] ? supdrvIOCtlFast+0x7f/0xa0 [vboxdrv]
Jun 18 08:35:12 localhost kernel: [ 1697.719540]  [<ffffffffa0374389>] ? VBoxDrvLinuxIOCtl_4_2_0+0x49/0x200 [vboxdrv]
Jun 18 08:35:12 localhost kernel: [ 1697.719543]  [<ffffffff8118c30a>] ? do_vfs_ioctl+0x8a/0x340
Jun 18 08:35:12 localhost kernel: [ 1697.719546]  [<ffffffff810a1f37>] ? sys_futex+0x147/0x1a0
Jun 18 08:35:12 localhost kernel: [ 1697.719548]  [<ffffffff8118c651>] ? sys_ioctl+0x91/0xa0
Jun 18 08:35:12 localhost kernel: [ 1697.719551]  [<ffffffff816686c2>] ? system_call_fastpath+0x16/0x1b
Jun 18 08:35:12 localhost kernel: [ 1697.719553] Code: 85 f6 74 45 85 c9 48 8d 75 b8 75 1b 48 c7 c6 40 43 01 00 65 48 03 34 25 50 da 00 00 f6 46 20 01 74 0b 0f 1f 00 f3 90 f6 46 20 01 <75> f8 66 c7 46 20 01 00 0f ae f0 48 89 56 18 4c 89 66 10 89 ca 
Jun 18 08:35:12 localhost kernel: [ 1697.719577] Call Trace:
Jun 18 08:35:12 localhost kernel: [ 1697.719583]  [<ffffffffa03845ab>] VBoxHost_RTMpPokeCpu+0x4b/0x60 [vboxdrv]
Jun 18 08:35:12 localhost kernel: [ 1697.719588]  [<ffffffff810a0516>] ? futex_wake_op+0x3b6/0x520
Jun 18 08:35:12 localhost kernel: [ 1697.719594]  [<ffffffffa0377bcf>] ? supdrvIOCtlFast+0x7f/0xa0 [vboxdrv]
Jun 18 08:35:12 localhost kernel: [ 1697.719600]  [<ffffffffa0374389>] ? VBoxDrvLinuxIOCtl_4_2_0+0x49/0x200 [vboxdrv]
Jun 18 08:35:12 localhost kernel: [ 1697.719603]  [<ffffffff8118c30a>] ? do_vfs_ioctl+0x8a/0x340
Jun 18 08:35:12 localhost kernel: [ 1697.719605]  [<ffffffff810a1f37>] ? sys_futex+0x147/0x1a0
Jun 18 08:35:12 localhost kernel: [ 1697.719607]  [<ffffffff8118c651>] ? sys_ioctl+0x91/0xa0
Jun 18 08:35:12 localhost kernel: [ 1697.719609]  [<ffffffff816686c2>] ? system_call_fastpath+0x16/0x1b