I also have a similar ( if not identical bug). Occurs under heavy load. Has been ongoing for several versions. Lockup is always preceded by an identical NetworkManager error.

Jul 24 14:52:20 gerbal-laptop NetworkManager: <info>  Error getting killswitch power: org.freedesktop.DBus.Error.NoReply - Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken. 
Jul 24 14:52:26 gerbal-laptop kernel: [13162.463932] BUG: soft lockup - CPU#1 stuck for 11s! [Xorg:5801]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.463942] 
Jul 24 14:52:26 gerbal-laptop kernel: [13162.463946] Pid: 5801, comm: Xorg Tainted: P        (2.6.24-19-generic #1)
Jul 24 14:52:26 gerbal-laptop kernel: [13162.463951] EIP: 0060:[<f904e633>] EFLAGS: 00203283 CPU: 1
Jul 24 14:52:26 gerbal-laptop kernel: [13162.464109] EIP is at _ZN4Asic16Is_WPTR_equ_RPTR19ConditionSuccessfulEv+0x33/0x50 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.464115] EAX: f8e8c080 EBX: eefe7d98 ECX: f95caf00 EDX: f93ba020
Jul 24 14:52:26 gerbal-laptop kernel: [13162.464120] ESI: f8e8c084 EDI: eefe7d98 EBP: eefe7d00 ESP: eefe7ce8
Jul 24 14:52:26 gerbal-laptop kernel: [13162.464125]  DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
Jul 24 14:52:26 gerbal-laptop kernel: [13162.464129] CR0: 8005003b CR2: b695c000 CR3: 1f8dc000 CR4: 00000690
Jul 24 14:52:26 gerbal-laptop kernel: [13162.464134] DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
Jul 24 14:52:26 gerbal-laptop kernel: [13162.464139] DR6: ffff0ff0 DR7: 00000400
Jul 24 14:52:26 gerbal-laptop kernel: [13162.464170]  [<f904d6e8>] _ZN4Asic9WaitUntil15WaitForCompleteEv+0x38/0xf0 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.464336]  [<f9051786>] _ZN6AsicR616ASICIdleInternalEN4Asic15idle_WaitMethodE+0x96/0x1f0 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.464549]  [<f902f4b6>] QSSubmitList+0x146/0x150 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.464698]  [<f904bf9c>] _ZN4Asic7PM4idleENS_15idle_WaitMethodE+0x4c/0x80 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.464858]  [unix_stream_recvmsg+0x25d/0x550] unix_stream_recvmsg+0x25d/0x550
Jul 24 14:52:26 gerbal-laptop kernel: [13162.464868]  [<c0125f20>] default_wake_function+0x0/0x10
Jul 24 14:52:26 gerbal-laptop kernel: [13162.464887]  [<f9046645>] _ZN15QS_PRIVATE_CORE7PM4idleEN4Asic15idle_WaitMethodE+0x35/0x70 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.465051]  [<f9036321>] _ZN10QS_PRIVATE11synchronizeEv+0x31/0x40 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.465204]  [<f902f4d7>] QSSynchronize+0x17/0x20 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.465352]  [<f9036e69>] _Z14uQSSynchronizej+0x19/0x20 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.465504]  [<f903e49f>] _Z8uCWDDEQCjjjPvjS_+0x33f/0x11a0 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.465670]  [<f902edf4>] CMMQS_uCWDDEQC+0x34/0x40 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.465822]  [<f8ff5e48>] firegl_cmmqs_CWDDE_32+0x238/0x340 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.465951]  [sys_quotactl+0x4e6/0x680] sys_quotactl+0x4e6/0x680
Jul 24 14:52:26 gerbal-laptop kernel: [13162.465962]  [<f8ff4e06>] firegl_cmmqs_CWDDE32+0x76/0x110 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.466097]  [<f8ff4d90>] firegl_cmmqs_CWDDE32+0x0/0x110 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.466218]  [<f8fe74be>] firegl_ioctl+0x19e/0x220 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.466333]  [sys_quotactl+0x4e6/0x680] sys_quotactl+0x4e6/0x680
Jul 24 14:52:26 gerbal-laptop kernel: [13162.466353]  [sys_quotactl+0x4e6/0x680] sys_quotactl+0x4e6/0x680
Jul 24 14:52:26 gerbal-laptop kernel: [13162.466361]  [<f8fdc85c>] ip_firegl_ioctl+0x1c/0x30 [fglrx]
Jul 24 14:52:26 gerbal-laptop kernel: [13162.466472]  [sys_quotactl+0x4e6/0x680] sys_quotactl+0x4e6/0x680
Jul 24 14:52:26 gerbal-laptop kernel: [13162.466484]  [do_ioctl+0x78/0x90] do_ioctl+0x78/0x90
Jul 24 14:52:26 gerbal-laptop kernel: [13162.466499]  [vfs_ioctl+0x22e/0x2b0] vfs_ioctl+0x22e/0x2b0
Jul 24 14:52:26 gerbal-laptop kernel: [13162.466515]  [sys_ioctl+0x56/0x70] sys_ioctl+0x56/0x70
Jul 24 14:52:26 gerbal-laptop kernel: [13162.466529]  [sysenter_past_esp+0x6b/0xa9] sysenter_past_esp+0x6b/0xa9
Jul 24 14:52:26 gerbal-laptop kernel: [13162.466539]  [sys_quotactl+0x4e6/0x680] sys_quotactl+0x4e6/0x680
Jul 24 14:52:26 gerbal-laptop kernel: [13162.466555]  [unix_stream_sendmsg+0x160/0x390] unix_stream_sendmsg+0x160/0x390
Jul 24 14:52:26 gerbal-laptop kernel: [13162.466572]  =======================