please download
http://www.stvsoft.com/SensorsView12Setup.exe
install it and post a screen shot of the temeprature readings and voltages on idle and load.
what directx are you running ? have you updated it recently ?
please as tiber suggested try the 84.21 drivers from nvidia as those are the most stable ones especially for the 6800 ultra.
pleas also run through this :
http://extreme.outervision.com/psucalculator.jsp and add 30% of the total to it. if you also could please post your processor.