For the past couple of months I've been having crashes every 30 minutes to an hour in TF2, and I've had this in Alien Swarm a couple times as well. About a month ago my computer crashed to a blue screen twice, and after the second time Windows was unable to boot and I couldn't repair it through startup recovery. I wiped, but the issues with Source games have persisted.

Earlier today on Granary in TF2, the colour of the screen was rapidly flickering and shadows have become huge black squares. I immediately fired up SpeedFan but temperatures weren't abnormally high, as both my GPU and CPU were comfortably between 50C-60C. They usually idle in the high 40s.

Here's a couple pictures I took of what happened:
http://imgur.com/ZhdID.jpg
http://imgur.com/q1OzD.jpg
http://imgur.com/xa0UV.jpg

My guess is that it has something to do with corrupt memory. I have Memtest burnt onto a DVD but I'm not really sure how to use it, or how long I should run it for?

My specs:
Intel E5200 2.5ghz Dual Core
2 x 1gigs Corsair RAM (2gb in total)
Asus P5KC Mobo
XFX Nvidia 9600GT 512mb

Try running furmark or some other graphics intensive application to check if your GPU is functioning fine.

I would be looking at GPU or possibly strained PSU as culprits for that behaviour. Can you confirm if this has happened in other games.

Thanks K3D. Haven't seen it in any other games, although I haven't really been playing many games other than TF2 lately.

Running the stress test program now, the GPU seems to have peaked at 77 degrees with no visible artefacts. How long should I run it for?

erm 30 mins should have been fine.

I would run orthos prime to check system stability.. at least 4 passes on the stress test.

Also verify your steam cache for TF2 and see if it re-aqcuires some files.. this could be an indicator of corruption.

Really need to find out if its a source engine issue or a PC problem.

Verifying the cache for TF2 turned up nothing. Was working fine when I joined the server, then after a few minutes it just went crazy. Closing TF2 for a bit then running Dustbowl in offline play worked fine.

I'll give the other things you said a go tomorrow.

This just happened to me

I may re install tf2 or de fragment + cache hope its just tf2 it appears to be so far...
I played empire total war, not bugs there, must be tf2 textures i presume or sam's gpu

EDIT* I only have big blackness on control points and others, not on the shadows or those colours, appears to be textures for me.