Happens about once a week, but I can't reproduce it at will. X is still running as before, but unbearably slow. GPU is an integrated HD4200 of an AMD 785G ASUS mainboard, model M4A785D-M Pro.

[261062.255333] [TTM] Erroneous page count. Leaking pages.
[261062.753541] BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
[261062.753705] IP: [<ffffffff813f9a71>] ttm_bo_cleanup_memtype_use+0x51/0x80
[261062.753830] PGD 214e12067 PUD 214e64067 PMD 0 
[261062.753918] Oops: 0000 [#1] SMP 
[261062.753981] CPU 0 
[261062.754007] Modules linked in: sata_sil
[261062.754007] 
[261062.754007] Pid: 2467, comm: xbmc.bin Not tainted 3.2.18 #1 System manufacturer System Product Name/M4A785D-M PRO
[261062.754007] RIP: 0010:[<ffffffff813f9a71>]  [<ffffffff813f9a71>] ttm_bo_cleanup_memtype_use+0x51/0x80
[261062.754007] RSP: 0018:ffff88021fbd7c58  EFLAGS: 00010256
[261062.754007] RAX: 0000000000000000 RBX: ffff88003c8c6c48 RCX: ffff8802267f0548
[261062.754007] RDX: 0000000000000000 RSI: ffff88003c8c6ca8 RDI: ffff8802267f0570
[261062.754007] RBP: 0000000000000002 R08: 0000000000000000 R09: ffffffff813f9a41
[261062.754007] R10: 0000000000000039 R11: 0000000000000246 R12: ffff8802267f0548
[261062.754007] R13: ffff8802259853c0 R14: ffff88003c8c6c8c R15: 00000000fffffff2
[261062.754007] FS:  00007fe3f3cba720(0000) GS:ffff88022fc00000(0000) knlGS:0000000000000000
[261062.754007] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[261062.754007] CR2: 0000000000000018 CR3: 0000000214e0d000 CR4: 00000000000006f0
[261062.754007] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[261062.754007] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[261062.754007] Process xbmc.bin (pid: 2467, threadinfo ffff88021fbd6000, task ffff880222120000)
[261062.754007] Stack:
[261062.754007]  ffff88003c8c6c48 ffffffff813fad00 ffff88004421b200 ffff88004421b200
[261062.754007]  ffff88021fbd7d74 0000000000000000 ffff880226ff5000 ffff88003c8c6c88
[261062.754007]  ffffffff813faaa0 000000000000000a ffff88003c8c6e00 ffff880226ff5000
[261062.754007] Call Trace:
[261062.754007]  [<ffffffff813fad00>] ? ttm_bo_release+0x260/0x280
[261062.754007]  [<ffffffff813faaa0>] ? ttm_bo_delayed_workqueue+0x30/0x30
[261062.754007]  [<ffffffff813304e3>] ? kref_put+0x33/0x70
[261062.754007]  [<ffffffff813f9ec5>] ? ttm_bo_unref+0x35/0x50
[261062.754007]  [<ffffffff8142f902>] ? radeon_bo_unref+0x42/0x80
[261062.754007]  [<ffffffff813e4f60>] ? drm_gem_object_ref_bug+0x10/0x10
[261062.754007]  [<ffffffff8143fc1f>] ? radeon_gem_object_free+0x1f/0x30
[261062.754007]  [<ffffffff813304e3>] ? kref_put+0x33/0x70
[261062.754007]  [<ffffffff813e50c8>] ? drm_gem_handle_delete+0xa8/0xf0
[261062.754007]  [<ffffffff813e374c>] ? drm_ioctl+0x3ec/0x4a0
[261062.754007]  [<ffffffff8132d54d>] ? cpumask_any_but+0x2d/0x40
[261062.754007]  [<ffffffff813e55f0>] ? drm_gem_destroy+0x50/0x50
[261062.754007]  [<ffffffff810cda5e>] ? tlb_finish_mmu+0xe/0x50
[261062.754007]  [<ffffffff810d468b>] ? unmap_region+0x10b/0x150
[261062.754007]  [<ffffffff8110a7b6>] ? do_vfs_ioctl+0x96/0x540
[261062.754007]  [<ffffffff810d4562>] ? remove_vma+0x52/0x70
[261062.754007]  [<ffffffff810d5936>] ? do_munmap+0x2d6/0x360
[261062.754007]  [<ffffffff8110aca9>] ? sys_ioctl+0x49/0x80
[261062.754007]  [<ffffffff8175e73b>] ? system_call_fastpath+0x16/0x1b
[261062.754007] Code: 00 00 00 00 00 00 48 83 7b 60 00 48 8b 4b 08 8b 93 84 00 00 00 74 17 89 d2 48 8d 73 60 48 c1 e2 07 48 8b 44 11 50 48 8d 7c 11 28 <ff> 50 18 c7 83 20 01 00 00 00 00 00 00 48 8d 7b 48 31 c9 31 d2 
[261062.754007] RIP  [<ffffffff813f9a71>] ttm_bo_cleanup_memtype_use+0x51/0x80
[261062.754007]  RSP <ffff88021fbd7c58>
[261062.754007] CR2: 0000000000000018
[261062.795877] ---[ end trace 4bb94fa28b7f787a ]---