Alright, I don't know if you covered this or not, but i read upto 10 pages of posts. I had the same problem with CSS. When i first got my old computer, It was outfitted with a Asus P4p800 SE, P4 3.00 Ghz Prescott, an ATI X700 Pro AGP, 1 GB DDR-400 Ram, and a 450 W PS. Now, when i first got counter strike It all would run great, 80+ FPS. But after awhile i noticed that my FPS were getting down into 20 and then would just cap off at 10 FPS, would not drop nor go up. I would have to restart my computer for it to work correctly again, and sometimes i wouldn't even get lucky and it would still do it. SO, after a little while I was getting fed up with it, so I reinstalled XP. Still did it after a clean install. NOW something more was going on when this did it. I was getting "Coil Whine" near the CPU area, Or called "Coil fluctuating" So i ran speed fan and CPUID and noticed that my Voltages on the CPU were fluctuating up and down while this "whine", or "hissing" sound was coming around the CPU area. What was happening was that the power supply was S**t and was giving out dirtty power to the motherboard, and this caused the regulators around the CPU to go bad. Eventually i replaced my mobo and got a good power supply. It fixed the problem.
EDIT: If you can, try swapping the CPU first