I've had this problem for a few months now. At first, I thought it was my PSU, and I got a new one. Didn't help at all. I could hardly play for 10 minutes on Normal textures + video memory, no anti aliasing, anistropic filter, no post processing, very low terrain, low objects detail, and high shadows.
For awhile now, I've been trying to play the game, on the lowest settings, 800 x 600 windowed, with the processor power set way down. Unfortunately, that doesn't help much at all. Recently, with the release of 1.54 and the BAF, I decided screw it, and tried the game at my normal settings, full screen 1440x900 resolution- and I was able to play for 6-7 hours STRAIGHT.
After I stopped playing, I decided to update ACE, but it was being dumb and screwing stuff up so I decided not to bother with it, and continued to play vanilla. Problem is right after that, my comp shut off after about 15 minutes. Right before I decided to post this, I tried one more time, and the game shut off after being at the main menu for not even 10 seconds.
What do? I get this feeling its my processor, because it's shut off while browsing the internet and playing TF2.