Why ubuntu freezes?

[oxy]

Hi all,

ubuntu 12.04 freezes when this msg appears in the mesg:

[22893.402669] radeon 0000:01:00.0: GPU lockup CP stall for more than 10056msec
[22893.402674] GPU lockup (waiting for 0x0008E2F0 last fence id 0x0008E2EE)
[22893.403849] radeon 0000:01:00.0: GPU softreset
[22893.403852] radeon 0000:01:00.0:   GRBM_STATUS=0xA27338A0
[22893.403855] radeon 0000:01:00.0:   GRBM_STATUS_SE0=0x1C000007
[22893.403857] radeon 0000:01:00.0:   GRBM_STATUS_SE1=0x1C000007
[22893.403860] radeon 0000:01:00.0:   SRBM_STATUS=0x200000C0
[22893.403872] radeon 0000:01:00.0:   GRBM_SOFT_RESET=0x00007F6B
[22893.403975] radeon 0000:01:00.0:   GRBM_STATUS=0x00003828
[22893.403978] radeon 0000:01:00.0:   GRBM_STATUS_SE0=0x00000007
[22893.403980] radeon 0000:01:00.0:   GRBM_STATUS_SE1=0x00000007
[22893.403983] radeon 0000:01:00.0:   SRBM_STATUS=0x200000C0
[22893.404978] radeon 0000:01:00.0: GPU reset succeed
[22893.443985] [drm] PCIE GART of 512M enabled (table at 0x0000000000040000).
[22893.444090] radeon 0000:01:00.0: WB enabled
[22893.460317] [drm] ring test succeeded in 3 usecs
[22893.460329] [drm] ib test succeeded in 3 usecs
[22903.578252] radeon 0000:01:00.0: GPU lockup CP stall for more than 10000msec
[22903.578257] GPU lockup (waiting for 0x0008E2FA last fence id 0x0008E2F9)
[22903.579464] radeon 0000:01:00.0: GPU softreset
[22903.579467] radeon 0000:01:00.0:   GRBM_STATUS=0xA27328A0
[22903.579470] radeon 0000:01:00.0:   GRBM_STATUS_SE0=0x1C000005
[22903.579473] radeon 0000:01:00.0:   GRBM_STATUS_SE1=0x1C000005
[22903.579476] radeon 0000:01:00.0:   SRBM_STATUS=0x200002C0
[22903.579501] radeon 0000:01:00.0:   GRBM_SOFT_RESET=0x00007F6B
[22903.579605] radeon 0000:01:00.0:   GRBM_STATUS=0x00003828
[22903.579608] radeon 0000:01:00.0:   GRBM_STATUS_SE0=0x00000007
[22903.579611] radeon 0000:01:00.0:   GRBM_STATUS_SE1=0x00000007
[22903.579614] radeon 0000:01:00.0:   SRBM_STATUS=0x200000C0
[22903.580609] radeon 0000:01:00.0: GPU reset succeed
...
... and it repeats several times ...

I use a radeon HD 6870 video card.
Ubu 12.04 64bit
Motherboard P8Z68-V LE.

No proprietary drivers being used. However there is this
option at system settings for ATI/AMD FGLRX proprietary
graphic drivers.

It may freeze several times a day!
I'd greatly thank for any help!

oxy