Hi, don't know if this will help?..
I had the same anomalies and hard reeboots with my Gainward gs 6800gt
so I took my case cover off to find that one of my fans on my 6800gt wasn't working!
so I emailed Overclockers UK for a RMA and they said I could RMA it.
But I found that sometimes the fan spins up and sometimes it dosn't,(so It was over heating)and when it
dosn't I just help it on its way with a quick flick of my finger (lol..sound's like I'm with the ex missus!)
So I havn't RMA'd it (can't do with out my lock-on!) and It works fine..gl ;)