Source Engine games have erratic FPS and CPU usage.

[Tacogeddon]

For a few weeks now, all of my Source Engine games have had this issue where the FPS cascades up and down, regardless of graphics settings or in-game action. TF2, for example, with graphics set as low as possible on -dxlevel 80, will be gliding along at 100+ FPS, then plummet to 15-20, then rocket back to 100+, and just repeat this endlessly, even if I just stare up the skybox on an empty map with no bots or players. My immediate assumption was that some background process was eating up my CPU, so I opened up task manager while running DOD:S windowed, and the usage was all over the place.

[URL]http://i.imgur.com/Ks5kL.jpg[/URL]

When the usage goes down, so does the performance. The problem is only afflicting Source Engine games, every other game and program on my system uses the CPU as expected. I've done the usual Source game steps, defragging this, verifying that, and I recently defragged my HDD and cleaned my PC of any viruses and the like. I also asked around the Steam forums, but didn't get much more than "it's probably the CPU."

[naelstrof]

When Source is windowed and backgrounded, it uses less CPU and drops frames. That's normal.

[BackSapper]

Well it is probably the CPU, Source games are very dependant on the processor. Please give specs.