Source Game Freezes

Hello all--

I am having a problem with my source engine games (Left 4 Dead and Team Fortress 2 are those I have tried so far). The game starts up normally, I can join games and play with no video or sound issues for about 2 minutes, at which point the game freezes up and becomes totally unresponsive.

What is really strange is that sometimes I can ctrl-alt-delete to get out of the frozen game, and then alt-tab back in, and it works for another 2 minutes or so and then locks up again. Sometimes the sound loops, and sometimes it is silent.

The details:
Windows 7 RC (7100) x64
AMD Athlon X2 6000+
2GB RAM
nVidia 8800GTX, 185.85 drivers
USB Headphones
USB Mic

Things I have tried, with no success:
Reinstalling the game
Running in windowed mode
Disabling multicore rendering in game settings
Running "mat_queue_mode 0" in console to force disabling of multicore rendering
Disabling onboard sound in BIOS
Updating DirectX
Windows Update

I don't think I have a hardware problem: these games run fine under Vista and XP on my system, and furthermore I can play Crysis on Win7 for as long as I want with no crashes. The issue seems to be localized to steam games using the source engine.

Any help would be greatly appreciated

I seem to have fixed it, in case anyone has the same problem. I alt-tabbed out of the game and manually changed processor affinity to CPU 0 only using the task manager. Not clear why this worked, considering that disabling multicore rendering in the game didn't, but oh well.