I'm having almost the same problem except my rig shuts down completely...I think my AIW X800 is overheating, so I ordered a new GPU cooler.- this MIGHT be what's going on in your case as well.
Just to be safe, I've also ordered a new PSU and Processor cooler...can't play any games until they get here and although they are here in town, I can't get them 'til Monday because they came in to the airport Fed-exed this morning....bums....