I heard in the latest podcast about someone (Mr. Ploppy?) getting sick playing Half-Life 2. I had the same problem.
Long Story short... try lowering your FPS to between 30 and 40 fps by adding +fps_max XX to your launch options on Steam for Source engine games. Mileage may vary, but worked PERFECTLY for me.
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The first time I noticed it was way back when I first got my Dell Pentium II 450 MHz desktop with a Diamond Viper video card and download a bunch of demos to test out my new rig. I was playing some version of Quake and noticed after about 10 minutes I wasn't feeling well. 30 minutes later and I was laid out on the couch sick as a dog. I assumed it was the flu or food poisoning. Later in the week I played again and got the same feeling and realized it was in fact the game.
I didn't really understand. I played those kind of first-person shooters before (if they were even called that back then) and never had an issue. Years later, I had played quite a few FPSs and never had an issue until my wife bought me the Orange Box and I fired up Half-Life 2. I felt the sickness coming on and stopped. The strange thing was I played a ton of Team Fortress 2 and felt just fine. I tried changing the FOV, aspect ratios and distance from the monitor—nothing worked.
Then I played TF2 on a different computer and I felt sick again. The difference: frame rate. I got sick with TF2 on the better machine which played the game at a much higher frame rate (60-ish fps). I found a setting in launch options to set the FPS max and set it to 30. No more sickness. I upped it to 40 and still felt fine. Anything higher and I am doomed.
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I know, I know. 30 fps?!? That's crap! Well, not really. Movie frame rates are 24 fps normally. It's a matter of taste in my opinion. But in the case of FPSs, if I can't lower it, I can't play it. I'd rather play it.
Hopefully this helps someone else out there. If anyone has anything else that worked for them, might as well post it to this thread.