You can monitor your GPU temperature easily with a program called "gpu-z" here is there website: http://www.techpowerup.com/gpuz/ Navigate to the "sensors" tab and you can see the temperature. leave that program open when you play a game and simply click the temperature box to change from current, min, max, and average temperature. @95C the video card starts to throttle clocks down to reduce heat and avoiding any possible damage to the card, or in your case shutting the laptop off.

I've noticed on a lot of consoles that the tolerance is not very tight for the heat sinks sitting on top of cpu/gpu. Maybe after a few heat cycles, the thermal compound has dried and created small gaps. One fix that I recommend(as long as your sure it is still an over heating issue) is to purchase copper cpu plates from Ebay and install on the two large cpu/gpu blocks. this will give a tighter tolerance, ensuring that the heat sink will be making full contact with both processing units(cpu and gpu). They make plates specifically for ps3 and you'll find they are pretty cheap! Measurements for copper plates should be roughly 42mm x 42mm and at least 1.2mm thick.