Not sure it is related to actual freezing incidents, but I saw this in kernel log at suspend:

Apr  2 09:46:26 lhs kernel: [  305.963399] ------------[ cut here ]------------
 Apr  2 09:46:26 lhs kernel: [  305.963436] WARNING: CPU: 2 PID: 2892 at /build/buildd/linux-3.13.0/drivers/gpu/drm/i915/intel_uncore.c:125 gen6_gt_check_fifodbg.isra.9+0x38/0x50 [i915]()
 Apr  2 09:46:26 lhs kernel: [  305.963437] MMIO read or write has been dropped ffffffff
 Apr  2 09:46:26 lhs kernel: [  305.963488] Modules linked in: ctr ccm snd_hda_codec_hdmi snd_hda_codec_realtek joydev dell_wmi sparse_keymap pn544_mei mei_phy pn544 hci nfc ppdev intel_rapl x86_pkg_temp_thermal intel_powerclamp cor
etemp dell_laptop dcdbas kvm_intel arc4 kvm crct10dif_pclmul crc32_pclmul ghash_clmulni_intel aesni_intel aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd psmouse serio_raw btusb wmi bnep rfcomm bluetooth iwlmvm mac80211 snd_h
da_intel snd_hda_codec snd_hwdep uvcvideo snd_pcm iwlwifi videobuf2_vmalloc videobuf2_memops videobuf2_core snd_page_alloc i915 snd_seq_midi snd_seq_midi_event snd_rawmidi parport_pc videodev video cfg80211 snd_seq snd_seq_device dr
m_kms_helper snd_timer drm lpc_ich snd mac_hid mei_me i2c_algo_bit mei soundcore lp nls_iso8859_1 parport e1000e ahci libahci ptp pps_core
 Apr  2 09:46:26 lhs kernel: [  305.963491] CPU: 2 PID: 2892 Comm: kworker/u8:12 Not tainted 3.13.0-21-generic #43-Ubuntu
 Apr  2 09:46:26 lhs kernel: [  305.963492] Hardware name: Dell Inc. Latitude E7440/07F3F4, BIOS A08 02/18/2014
 Apr  2 09:46:26 lhs kernel: [  305.963515] Workqueue: i915 gen6_force_wake_work [i915]
 Apr  2 09:46:26 lhs kernel: [  305.963520]  0000000000000009 ffff8803c2935d38 ffffffff81714914 ffff8803c2935d80
 Apr  2 09:46:26 lhs kernel: [  305.963523]  ffff8803c2935d70 ffffffff810676bd ffff880406c48020 ffff880406c48028
 Apr  2 09:46:26 lhs kernel: [  305.963526]  0000000000000246 0000000000000000 0000000000000100 ffff8803c2935dd0
 Apr  2 09:46:26 lhs kernel: [  305.963527] Call Trace:
 Apr  2 09:46:26 lhs kernel: [  305.963534]  [<ffffffff81714914>] dump_stack+0x45/0x56
 Apr  2 09:46:26 lhs kernel: [  305.963538]  [<ffffffff810676bd>] warn_slowpath_common+0x7d/0xa0
 Apr  2 09:46:26 lhs kernel: [  305.963540]  [<ffffffff8106772c>] warn_slowpath_fmt+0x4c/0x50
 Apr  2 09:46:26 lhs kernel: [  305.963547]  [<ffffffff8109a65a>] ? try_to_wake_up+0x1fa/0x2c0
 Apr  2 09:46:26 lhs kernel: [  305.963568]  [<ffffffffa02b6de8>] gen6_gt_check_fifodbg.isra.9+0x38/0x50 [i915]
 Apr  2 09:46:26 lhs kernel: [  305.963585]  [<ffffffffa02b6e8b>] __gen6_gt_force_wake_mt_put+0x2b/0x30 [i915]
 Apr  2 09:46:26 lhs kernel: [  305.963602]  [<ffffffffa02b6bb7>] gen6_force_wake_work+0x37/0x50 [i915]
 Apr  2 09:46:26 lhs kernel: [  305.963606]  [<ffffffff81083892>] process_one_work+0x182/0x450
 Apr  2 09:46:26 lhs kernel: [  305.963610]  [<ffffffff81084631>] worker_thread+0x121/0x410
 Apr  2 09:46:26 lhs kernel: [  305.963614]  [<ffffffff81084510>] ? rescuer_thread+0x3e0/0x3e0
 Apr  2 09:46:26 lhs kernel: [  305.963617]  [<ffffffff8108b302>] kthread+0xd2/0xf0
 Apr  2 09:46:26 lhs kernel: [  305.963620]  [<ffffffff8108b230>] ? kthread_create_on_node+0x1d0/0x1d0
 Apr  2 09:46:26 lhs kernel: [  305.963624]  [<ffffffff8172523c>] ret_from_fork+0x7c/0xb0
 Apr  2 09:46:26 lhs kernel: [  305.963627]  [<ffffffff8108b230>] ? kthread_create_on_node+0x1d0/0x1d0
 Apr  2 09:46:26 lhs kernel: [  305.963629] ---[ end trace 396fb56d04ab9b14 ]---
 Apr  2 09:46:26 lhs kernel: [  306.542815] PM: suspend of devices complete after 599.849 msecs
 Apr  2 09:46:26 lhs kernel: [  306.543013] PM: late suspend of devices complete after 0.196 msecs