Something rings a bell about that GPU. But ask the person who works on your laptop to use the canned air and possibly replace the heatsink compound on all heat sinks.
I have been experiencing a peculiar problem with my mac book Pro 15". the details of which are as follows:-
Mac OS X version 10.6.3
Processor 2.66GHz Intel Core i7
Memory 4GB 1067MHz DDR3
Intel HD Graphics -in built
NVIDIA Ge Force GT 330M PCLe
When using apps like i Movies, CS3 and while accessing my Hotmail account, the Mac just switches off, i.e. screen goes blank (black), keyboard lights come on and the fan is running. By holding down the power button the Mac can be switched off and restarted again.
Otherwise the Mac runs fine with no issues even while running apps like office, and watching movies or listening to music on itunes. I have tried replacing the Memory and Hard Disk but the problem is still there.
Would be really great full for some help on this.