Why does my PS3 keep freezing up.

I can turn the console on and do everything regularly for an hour or so, and then the console freezes, forcing me to hold down the power button to do anything. The fans stay calm this entire time, and a frozen picture is displayed on the screen. The console can even do this, if I just wait it out on the XMB. I’ve already taken apart the PlayStation and cleaned it out of any dust that could cause overheating, and I already formatted the HDD to make sure it wasn’t a corrupted OS. I’m at a loss for what it could be. Possibly an old HDD? In need of new thermal paste? IDK. Any help is awesome! Thanks!