Vaio FW Black screen

Okay, so here's my problem. I have a Vaio FW, with an ATI MR 4650 with 1 gb DDR3. I've been having an annoying problem since I first got the laptop. Basically, the screen will go black, and after a couple of minutes, the sound will start sounding distorted and weird. The only way to fix this is doing a hard restart. I had these problem with these specific games:
- Crysis / Crysis Warhead / Far Cry 2 / Mirror's Edge / Mass Effect / Batman: Arkham Asylum / The Saboteur / Assassin's Creed.
Games I had no problems with:
- Splinter Cell Conviction / GTA IV / The Witcher: EE / Fallout 3 / Dead Space / Just Cause 2 / Aliens vs. Predator (2010)

A couple of things you should know:
+ I fixed Crysis by switching to DX9, the crashes stopped completely
+ Changing detail settings to low changed nothing in most games, still crashed.
+ The easiest solution I could find was underclocking the card by 50 Mhz (both memory and clock)
+ At first I thought it was an overheating problem, or maybe a defective gpu, but I ran a FurMark stability test (1280x1024, 4XAA) for over two hours with no crashes.
+ The crashes are somehow related to the Catalyst drivers. For example, if I use modded Catalyst 9.12 drivers, Crysis DX9 will run just fine. But if I install >10.5, the game crashes pretty quickly.