I did not open the PS3 up. I used an air compressor (minimum psi) combined with a vacuum cleaner to blow every accessible vent clean and the PS3 is kept in space that is regularly cleaned.
I don't really like this "vacuum cleaner" idea because it could damage the fans fragile motor, people do that when they don't want too void the warranty. Since your ps3 is five years old the warranty is gone. My first fat ps3 and my new slim had the same problem(randomly shutting down) and every time the fix was to open them up and clean out the dust on the fan vents(except when my fat ps3 was ylod-ing)#
#When my fat ps3 was ylod-ing it was randomly shutting down but it was progressively getting worse and worse(shutting down once a day to multiple times a day) til it died then I fix it by heating up the motherboard whit a heat gun as to melt the solder under the cpu and gpu it lasted for about two months and then ylod-ed again so i fix it again and that time it lasted three weeks then I caved in and bought a new slim
In my opinion you should just open it up and see if it is clean and if it is then trade it in but if it ylod's before you can trade it in then get a heat gun and fix it, then trade it in.