sounds like a power problem . make sure all cards are seated and memory too. Do you have any other vid cards to try out or even psu . 450 seems a little under powered for a 9800 imo.
NEVER NEVER NEVER take the cpu cooler off , it only takes 2-4 seconds for that sucker to cook and be done
What thermal compound/TIM are you using between the cpu and cooler?
Lower your overclock
just a few things I would try