Those temps are okay. I can almost garuntee you it's your motherboard. I had the same board and games would work on low settings sometimes on med depending on the game. When the game was on high it would lock up and I would see artifacts. When I had my 9600xt the game would load and the screen would go black.
Try downloading you latest chipset drivers. That might help you.
These are the right drivers for you. So download and install these and see if that helps you.
If that doesn't work your next option is to buy a new motherboard.