Look for a guide on reapplying thermal paste to your board. Even with PCs, thermal paste breakdown happens after a few years. There are a lot of resources online about how to resolve an overheating issue with the PS3 based on all of the models, a quick google search will get you everything you need to know.
You risk permanent damage every time you let it force itself off. Though if you're determined it's not, back your saves up and reformat/restore. If it still happens after factory reset it's overheating.
I recommend ordering some Arctic Silver, there are two schools of high performance thermal paste in the overclocking world right now, one is silver compound the other is ceramic based. I have always found the silver compound to perform better and last longer, but that's just my opinion.
Edit: After a quick search I found a site with a bunch of good info.
Here
And to contradict myself the guide recommends this:
"Personally I went with the IC Diamond 7 (IC7) thermal compound. It is comprised of 92% micronized diamonds. Diamond is one of the best thermal compounds out there, and supposedly has 5 times better thermal conductivity as silver. It's cure time is only about 2 hours, and it costs about the same as AS5." [AS5 being arctic silver]