if its a thermaltake tr there 750 and 800w psu's werent very well made and often didnt meet there stated output in some cases by a long way. so that may be the rout of your problem if its hardware based.
make sure you also have hpet mode set in bios to match your o.s. 64 for 64 bit and 32 for 32bit. as this can cause lockups.
next check the event logs. make sure its not a software problem.
go into bios and make sure you load the optimized defaults. this wil reset the dma data incase that has been corrupted.
next run the stress test above links...
check your temps. gpu-z and core temp will do the job, make sure you log ythe temp's and check the vrm as well as the core temp of the gpu.
update your drivers including the intel nfo chipset driver (you may have to run this in vista compatability mode on win 7 64)