Im having the same problem. I have an ATI Radeon™ HD 2400 Pro AGP card [RV610].

Sometimes occur when the graphics are loading, as in this case.

And sometimes occurs in Unity desktop (like bug #774909).

dmesg

[ 67.512038] radeon 0000:01:00.0: GPU lockup CP stall for more than 10000msec
[ 67.512045] ------------[ cut here ]------------
[ 67.512092] WARNING: at /build/buildd/linux-2.6.38/drivers/gpu/drm/radeon/radeon_fence.c:248 radeon_fence_wait+0x2ea/0x350 [radeon]()
[ 67.512097] Hardware name: P4V88
[ 67.512100] GPU lockup (waiting for 0x000006CA last fence id 0x000006C6)
[ 67.512103] Modules linked in: parport_pc ppdev lp dm_crypt parport snd_via82xx gameport snd_ac97_codec ac97_bus snd_pcm binfmt_misc snd_page_alloc snd_mpu401_uart snd_seq_midi snd_rawmidi snd_seq_midi_event arc4 rt61pci crc_itu_t snd_seq rt2x00pci rt2x00lib psmouse snd_timer mac80211 snd_seq_device joydev serio_raw snd cfg80211 i2c_viapro soundcore shpchp eeprom_93cx6 squashfs aufs nls_iso8859_1 nls_cp437 vfat fat dm_raid45 xor btrfs zlib_deflate libcrc32c radeon usbhid ttm hid drm_kms_helper drm usb_storage uas i2c_algo_bit via_rhine sata_via pata_via
[ 67.512168] Pid: 3025, comm: compiz Not tainted 2.6.38-8-generic #42-Ubuntu
[ 67.512171] Call Trace:
[ 67.512185] [<c104f912>] ? warn_slowpath_common+0x72/0xa0
[ 67.512210] [<f828366a>] ? radeon_fence_wait+0x2ea/0x350 [radeon]
[ 67.512234] [<f828366a>] ? radeon_fence_wait+0x2ea/0x350 [radeon]
[ 67.512240] [<c104f9e3>] ? warn_slowpath_fmt+0x33/0x40
[ 67.512263] [<f828366a>] ? radeon_fence_wait+0x2ea/0x350 [radeon]
[ 67.512273] [<c106d370>] ? autoremove_wake_function+0x0/0x50
[ 67.512296] [<f8283ed1>] ? radeon_sync_obj_wait+0x11/0x20 [radeon]
[ 67.512309] [<f80e1719>] ? ttm_bo_wait+0xd9/0x170 [ttm]
[ 67.512320] [<f80e3dc2>] ? ttm_bo_move_accel_cleanup+0x162/0x1e0 [ttm]
[ 67.512344] [<f82841aa>] ? radeon_move_blit.clone.0+0x13a/0x1b0 [radeon]
[ 67.512368] [<f8284465>] ? radeon_move_ram_vram.clone.2+0x105/0x120 [radeon]
[ 67.512378] [<f80e19f2>] ? ttm_bo_list_ref_sub+0x22/0x30 [ttm]
[ 67.512388] [<f80e0072>] ? ttm_tt_set_caching+0x2/0x1f0 [ttm]
[ 67.512410] [<f828462a>] ? radeon_bo_move+0x1aa/0x1c0 [radeon]
[ 67.512420] [<f80e21c0>] ? ttm_bo_unmap_virtual_locked+0x50/0x60 [ttm]
[ 67.512443] [<f8284480>] ? radeon_bo_move+0x0/0x1c0 [radeon]
[ 67.512453] [<f80e22fd>] ? ttm_bo_handle_move_mem+0x12d/0x380 [ttm]
[ 67.512477] [<f8285930>] ? radeon_bo_move_notify+0x0/0x30 [radeon]
[ 67.512487] [<f80e3195>] ? ttm_bo_move_buffer+0x125/0x140 [ttm]
[ 67.512498] [<f80e19f2>] ? ttm_bo_list_ref_sub+0x22/0x30 [ttm]
[ 67.512506] [<f80e0072>] ? ttm_tt_set_caching+0x2/0x1f0 [ttm]
[ 67.512516] [<f80e3258>] ? ttm_bo_validate+0xa8/0x120 [ttm]
[ 67.512539] [<f8285533>] ? radeon_bo_list_validate+0x73/0xd0 [radeon]
[ 67.512567] [<f829c619>] ? radeon_cs_parser_relocs+0x209/0x250 [radeon]
[ 67.512594] [<f829c8a9>] ? radeon_cs_parser_init+0x249/0x2a0 [radeon]
[ 67.512621] [<f829ca25>] ? radeon_cs_ioctl+0x75/0x180 [radeon]
[ 67.512642] [<f80b4cd1>] ? drm_ioctl+0x1e1/0x470 [drm]
[ 67.512671] [<f829c9b0>] ? radeon_cs_ioctl+0x0/0x180 [radeon]
[ 67.512680] [<c1101627>] ? handle_mm_fault+0xb7/0x120
[ 67.512687] [<c150ce00>] ? do_page_fault+0x0/0x490
[ 67.512691] [<c150d02f>] ? do_page_fault+0x22f/0x490
[ 67.512705] [<f80b4af0>] ? drm_ioctl+0x0/0x470 [drm]
[ 67.512711] [<c11367cb>] ? do_vfs_ioctl+0x7b/0x2e0
[ 67.512716] [<c1136ab7>] ? sys_ioctl+0x87/0x90
[ 67.512721] [<c1054f02>] ? sys_gettimeofday+0x32/0x70
[ 67.512727] [<c1509bf4>] ? syscall_call+0x7/0xb
[ 67.512732] [<c1500000>] ? print_cpu_info+0xe3/0x126
[ 67.512735] ---[ end trace 88331dc8f6844363 ]---
[ 67.512749] [drm] Disabling audio support
[ 67.513780] radeon 0000:01:00.0: f4648800 unpin not necessary
[ 67.513787] radeon 0000:01:00.0: GPU softreset
[ 67.513791] radeon 0000:01:00.0: R_008010_GRBM_STATUS=0xE57024E0
[ 67.513795] radeon 0000:01:00.0: R_008014_GRBM_STATUS2=0x00110103
[ 67.513799] radeon 0000:01:00.0: R_000E50_SRBM_STATUS=0x200010C0
[ 67.513811] radeon 0000:01:00.0: R_008020_GRBM_SOFT_RESET=0x00007FEE
[ 67.528701] radeon 0000:01:00.0: R_008020_GRBM_SOFT_RESET=0x00000001
[ 67.544584] radeon 0000:01:00.0: R_008010_GRBM_STATUS=0xA0003030
[ 67.544589] radeon 0000:01:00.0: R_008014_GRBM_STATUS2=0x00000003
[ 67.544593] radeon 0000:01:00.0: R_000E50_SRBM_STATUS=0x200080C0
[ 67.545588] radeon 0000:01:00.0: GPU reset succeed
[ 67.565464] radeon 0000:01:00.0: WB disabled
[ 67.596797] [drm] ring test succeeded in 1 usecs
[ 67.596810] [drm] ib test succeeded in 1 usecs
[ 67.596814] [drm] Enabling audio support