your 6600gt is pci-e or agp? if agp did you connect the power cable to it? if not then better do so it have a major impact on its stability , also could be the psu. download prime95 & everest

http://www.mersenne.org/freesoft.htm

http://www.majorgeeks.com/download4181.html

run prime95 & everest (goto computer-->sensor) then start the stress testing in prime95 and while its doing it goto the everest window wait for 10-15mins and write down on paper the lowest the +3.3v/+5v/+12v gets then stop the stress testing in prime95 and close it & everest and report back with the voltages