Originally Posted by Hawke
Like grey_1 said your power supply could be on it's way out, to find out you will need to do some testing. You have already ruled out a software problem with a fresh install.
If your machine is overclocked reset your bios to its default settings, update the motherboards bios to the latest version and if it's still playing up see if you can borrow another PSU from a friend or another machine and use it to power the video card.
You don't need to remove your current PSU just unplug it from the video card, prop up the spare next to your case and plug it in, power it on first (use a paper clip to short the green pin and one of the black pins in the same row). Then turn on your machine and play some games.
At least that way you will know for sure if you need to go shopping for a new PSU.
It's not the PSU, it's practically new and has been running HD5870CF for the last three months with a 8800GT for Physx with zero problems.
I'm not going to troubleshoot hardware compatibilty as I've been there before, it's time consuming, expensive and usually a waste of time.
I'll just use my HD5870 for now and get a HD6970 next month.