Are you over clocking the processor or anything else ? If so you might be over doing it and/or don't have adequate cooling. Over heating would cause something like that to happen. Other issues like memory or a bad chip could do that too. Over heating is the most common issue though.
I noticed you have said you have 32 bit windows and 6 gigs of ram. 32 bit Windows cannot address more then 4 gigs of ram including the video ram.