have a 800w PSU, so cant lack of power
Are you sure its not the "over temperature" after all? Could be the settings are set too low in BIOS?
I dont know what OS you use, but windows have an event viewer, that might give you some clue...if it manaages to log it before shutting off that is.
Furmark stress test for 30min+FPSmin=64 max=635 average=82(temps went up-to 84 after about 10mins then stayed stable for the rest of the test at 84) - no crashIntelBurnTestV2Processor: AMD Phenom(tm) 9950 Quad-Core ProcessorClock Speed: 2.61 GHzActive Physical Cores: 4Total System Memory: 3327 MBStress Level: Maximum (930 MB)Testing started on 26/09/2010 20:04:05Time (s) Speed (GFlops) Result[20:08:45] 34.697 22.1286 3.376860e-002[20:14:04] 34.412 22.3115 3.376860e-002[20:19:28] 33.891 22.6547 3.376860e-002[20:24:49] 34.150 22.4827 3.376860e-002[20:30:11] 34.033 22.5600 3.376860e-002Testing ended on 26/09/2010 20:30:11Test Result: Success.