I've been playing Witcher 2 for a few hours, and up until now it has run fast with everything on high and ubersampling disabled. I'm running it on a gt240 overclocked to 680 Mhz core with 2100 Mhz shader. A little while ago, everything became really choppy. Thinking it was just due to higher load on the GPU, I bumped the settings way down, to no effect.

If I load earlier saves, they all run choppily - even when they did not before. For instance, at LaValette castle everything ran very smooth. Now when I load it, things are incredibly choppy. I have tried updating my drivers, uninstalling 3d vision and clearing out/defragmenting my hard drive.

Any ideas?

Try do decrease your settings, put put memory to normal, disable SSAO, disable motion blur, bloom, ubersampling, rain effects, DOF, and it will be fine.

Is this all guesswork, or have you actually used something like fraps to check the frame rate?

Some people experience choppiness despite having a fairly high frame rate, and a frame rate limiter like Dxtory sometimes fixes the problem.

Unless you've actually checked and lost FPS, in which case it's something totally different.

I have checked and lost. It's also very easy to see. Before it ran with no noticeable choppiness, while now it constantly freezes and stutters.

try remvoing all your saves except the most recent one

I tried but it didn't help. Down to three saves and still very laggy. Even if I turn settings down to low, the framerate doesn't change.

Problem resolved - seems simply deleting 3D vision drivers didn't do the trick. And reinstalling all new drivers over the others also didn't. Clean install bumped it up to ~50.