Aspire 5536 - Black screen, to do with GPU

Hello,

I have an Acer Aspire 5536 which has been well used but looked after for around two years now. I've recently had to replace my hard drive, but since then I've been having problems. Whenever I'm playing on a game or doing something requiring the GPU to act a little harder than normal, my screen will randomly go black. The power light, indicator lights and fan are all still on though.

I downloaded Netbook Hardware Control to check the laptop's temperatures, which were pretty high (approaching 100*C). So I disassembled the laptop to find there was virtually no cooling paste on either the CPU or GPU. I applied new paste, allowed it to set, cleaned the fan and vent and fired it back up. And although the temperatures were lowered quite substantially (50*C for hard drive, 70*C for CPU), the same thing keeps happening.

I then booted the laptop up in Safe Mode, which ran absolutely fine with no black screen appearing after hours of use. So through this I've deducted that there is a problem somewhere around the GPU.

One thing I noticed while applying the new thermal paste was the remains of what might have been a cooling pad, which I understand might be slightly thicker to ensure contact with the heat sink. When I reattached the fan and sink to the motherboard though, it seemed as if everything was in contact though.

Something which also keeps cropping up online is updating the BIOS. When I went to do this, it showed my current BIOS is the same version as the most recent downloadable on Acer's website.

Has anyone got a clue as to what's going on and how to fix this? I haven't tried reinstalling ATI drivers as of yet as I'm certain that won't make a difference (the display's fine).