Hey Michael. It sounds like your playstation is having some cooling issues. The PlayStation is probably shutting down as a fail safe to prevent damage from overheating.
I suggest taking the machine apart and cleaning it out. You would be surprised (grossed out) at the amount of dust that accumulates inside of game consoles, and that can be a serious detriment to cooling. While you are at it, pull the heat sinks off of the processors and replace the [product|IF179-010|thermal paste]. The factory thermal paste sometimes "dries out" and doesn't conduct heat as well as it used to. You can follow [guide|3493|this guide] to help you get to the processors.