COD5 performance under certain conditions

I am a veteran PC user with a very annoying problem

System
Q6600@3.6
2GIG
8800gt
2x sound cards (1 for recording)
Fully water cooled

VISTA HOME PREM latest updates

SOMETIMES when I fire up COD5 I have unplayable frame rate.
While this is happening adjusting video settings in game makes no difference at all.

The only way to get performance back up to normal is to reboot the PC.

It seems to be random to when the performance in COD5 drops, and i have checked things such as temps and speeds of GPU while this is happening.

At 1st i thought it was when i had been using my second sound card for a while, however it does not consistently kill by COD5 performance.

Today I turned PC on, went into COD5, had, Sub 40 frames, rebooted and was fine.

Other times ill use it all day, all my hardware, and have no issues.

I have the latest NV drivers and have tried various driver level quality settings.

I have recently found out its not just COD5 thats goes into "poor performance" mode.

Any game fired up while this happening is effected.

Its so annoying, the rest of the system is responsive as normal during this time


ANY IDEA WHY SUDDENLY PERFORMANCE DIES??

btw it never dies while im already in a game, if its fine when i 1st lauch it then it will be perfectly stable