Try playing other games that are quite cpu intensive for a while, not just mame I mean, other emus or pc games. My computer used to do that, turned out to be 2 capacitors on the motherboard, right next to the cpu, that looked like they were about to explode. Despite having a soldering iron powerful enough for my pc's motherboard, I managed to replace them with 2 new capacitors that cost me 1$ total, never had a problem since, and it's been over a year.

There used to be a bunch of faulty capacitors, I'm just saying it might be it, check them out, it's quite easy to see if they're busted.