I don't think the 5500(766) had sensors in the motherboard for the cpu. It'll show 75 no matter what...
With this model, the gpu is a "module", not a regular video card, so if yours is going bad you'll have to find a used one on eBay. They haven't made the modules for years.
It's likely your video module is overheating, and I'm not sure you can take it apart and apply AS5 or not.
The video modules were ATI MR 9600/9700, or geForce go 5600/5700. That is all that will work in the 766.