I'd take cypher's advice. Start backing up your data. You could call Sony customer service but they would charge you for a fix. It would probably be cheaper than buying a new PS3 though.
edit- Thermal paste might be an issue too. That would be the quickest and cheapest fix if that was the case. Since you can get thermal paste for about 10 dollars for a decent size tube (more than what you actually need). Perhaps open it up and check to see if the paste is dried up. Since your warranty is already void it wouldn't matter too much. Although you should make sure everything is backed up just in case something goes wrong while opening it up. I've actually applied paste a few times on my old computer and it takes only a few minutes if that.
