hmm id lean towards a power issue , what power supply you using , and have you got both power leads plugged in?
i have a 4870 and have run 3d mark on vista 64 without issue , and my temps have exceeded 80'c before i changed fan profile.
ok ive had issues with hipers before , might be worth testing it. before you had that card were you running nvidia? , if so did you remove all traces of the old drivers , often this can also cause a freezing issue. thats the only thing that i can think will cause it , possibly a complete o/s reinstall if viable could prove whether or not its driver based.
I tried to increase the GPU fan speed and the issue if not completly gone, it is greatly reduced. It seems to be a GPU issue after all. Good to know that the PSU is not the problem anyway. At least for now. But I cannot run my gpu fan on 40-50% all the time, it makes a lot of noice