well just now played for about 1 min, this time as i scoped in on cod4 and held my breath, my screen went into powesave, came out again, and i was at the desktop unable to resume. however, i did have the heartbeat constantly going which was quite cool.
Bum!
Now may be a good idea to do the desk fan test i suggested(?) - as you're treading water, somewhat, at the moment. You need to start getting your hands dirty

And if you're able to, it would be good if you could answer the OS questions i posed earlier as it would give us a clearer understanding into how your system is run.
If the desk top fan test doesn't yeild any results you may want to consider uninstalling/reinstalling all your core drivers.
EDIT: I've just noticed you have a 9600gt - if none of the above works i would definitely plug that in and see what happens.
EDIT x 2: If it does transpire the gfx is overheating, and your warranty allows it, andi's option 2 (medium) could shave a considerable amount off your temps - if not it will have been good practice for option 3 (hard). Although, you're probably technically minded enough now for both options to be relatively straight forward and re-classed as 'time consuming necessities'.
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