You’ll have to completely disassemble it all the way down until you can expose the CPU and GPU chip. Clean off the old dried up thermal paste off the chips and heat-sink and reapply your brand of choice. I’ve done this on my own PS3 as well as multiple laptops that did the same thing. Most boards have a temperature limit and will auto power down when the chips exceed the max temperature. It’s a safety measure design put into most boards so that they don’t self destruct.