Could be due to a possible overheat in your system. I had the same thing happen to my PC a few weeks ago while playing World of Warcraft .. (I would play normal for about the first 10 minutes and then everything would begin to get choppy) ... So I ran myself up to Radio Shack, and snagged myself a bottle of pressurized air. Came home and cleaned out my fans and heat sync's. After that everything was back to normal, WoW was running smoothly on "Ultra" graphics settings, with 58 - 60 FPS at all times.
Though before rushing out and buying canned air, I would recommend downloading and installing a program known as "SpeedFan" to check your GPU (Graphics card) + Your CPU's temperature! (Mine now stays at about 43c - 49c at all times, which is a normal temperature).
Once you install it, open "SpeedFan" and let it run. Then open Team Fortress 2 and play for about 15 / 20 minutes, then minimize, and check the temperature of those items.
Also just as a side note, according to an NVIDIA technician that I chatted with a few weeks ago: "The maximum temp on which the Graphics card can work is 65 to 70 C."
Other than that, it could be that your graphics card drivers are out of date, or that your trying to multitask while playing (such as having IE or Firefox open in the background). If so close them, as well as other unnecessary programs so that your not working your CPU harder than it has to.
Anyway, I hope this helped.
Reply back if you need anything else, and hopefully I'll be able to further assist you.
- Basmastersix =)
Edited by Basmastersix, 11 May 2010 - 11:40 PM.