According to Zeph, if your framerate is too high (yes, too high), the numbers in the counter will look good but the game can't keep up and will stutter and display errors. It has happened to me before. You can lower your settings to low or medium (on low, it looks almost as good as the xbox version) and then increase to your native resolution, and play with AA a bit until you can lower the framerate to between 30 and 70.

It's ridiculous how H2V doesn't have an option to lock the framerate at 30 fps like Halo PC did for the most consistent video performance. Your CPU, RAM, and video card are all superb (especially the video card, although the CPU is a bit dated). Just toy with settings until you can "lock" the framerate.

I run at 1680x1050 on lowest with 2x AA, with an Athlon X2 3800+, 1 GB RAM, and a Geforce 7600 GT (using Vista here). and I get around 40-70 fps which is consistent. On my laptop (2.13Ghz Centrino Solo, 2 GB RAM, Geforce 6600 and windows XP), I use 1280x720 on high without AA and I get 30 fps.