FPS for one of the worst gfx games ever (1.6)

I currently have a PentiumD 2.8 2 Core CPU along with 2gigs of 1066mhz Crucy ram, and the 8600GT nVidia card. When I bought the card it ran CS 1.6 so smooth through any situation in the game. But 2 days ago CS does something weird. It is running at a flat rate of 100FPS(common) and then every 15 sec it goes down to 15FPS for a millisecond, then back up to 100FPS for about 15more seconds. It does this non stop now and its annoying. Ive rolled back to old Drivers, updated drivers, reinstalled the card, tweeked the settings as much as I could in the nVidia control panel, and ive reinstalled CS countless amounts of times... I dont understand what the hell is happening! It could be a video card problem but then again it could be my CPU. Lately my CPU fan has went haywire(i guess do to my CPU heating up). Im not sure if the CPU usage being taken up, but everytime i start CS(or any application) the CPU fan starts going as hard as ive ever heard. Like i said im not sure the CPU usage is high when im in game but i do hear the fan like crazy.
I'm about to get a new motherboard for my e6850 intel CPU since i was thinking about upgrading, but before the mobo gets in does neone have any clues as to why CS is doing this?

Thanks.

It could be something spiking your CPU usage?