Couple of days ago my motherboard started to overheat its a Gigabyte 945P-S3 (Socket 775) my cpu gpu all stay under 40 degrees but my motherboard is always over 70 degrees and it shuts off when i start to play games it goes over a 100 degrees.I tried locating the heat source i opened the case but the motherboard was pretty cool so idk why speedfan and speccy show a massive heat.I read it might be a PSU problem
Which brand psu?
If you have a local brand psu then definitely the main reason is your psu because it might not working efficiently to provide full support to the system components.
Try changing your psu with the psus of corsair,seasonic,cool master for best performance regarding psu.
Please re-insert your components again and make sure that their is no dust on the vents , gpu, cpu cooler and motherboard components as well.Because sometimes improper contact with the components part lead to this problem.