Automatic restarts / shutdowns / freezing during games

[gusmthca]

HP Pavilion a6228x PC

500 watt PSU (upgraded)

9800 gtx (upgraded)

2x 2.6 ghz AMD proc

Basically my problem is that during internet browsing its fine. But when I enter a game (left4dead, crysis,) mostly newer games that it has run fine with in the past it will start to just shutdown or restart.... I have had the card for about 4 months and it only started this within the past month. I downloaded evga precision so i can monitor the card heat and fan speed to cool it better... and it hasent worked, I changed the drivers back to 182.50 when they were more stable and it hasent worked. I just upgraded them to 186, and its shutdown within 10 min of a game... and even if I use a weaker resolution and tone down the graphics as far as I can it will still act up and restart.  I am not sure if I dont have enough PSU power for it... the processor is too weak.. the motherboard cant handle the card or what.

[gusmthca]

also I heard the Motherboard fan might be too slow or something.. ive tried looking in f10 setup at startup but cant find the RPM