Game runs very nicely for me--no frame skipping that I can see, and I'm running at 1920x1080, max IQ settings all around except, of course, for Ubersampling, which is turned off. The game runs at an average of 25-35 fps, with occasional dips and spikes, as noted when I ran Fraps. My hardware specs are lower than yours:
Athlon X2 265 @ 2.9GHz
4GB DDR 2 @800MHz
ATi 5770 1GB running at slightly overclocked frequency of 900MHz/1300MHz (gpu/onboard ram), running 11.8 CATs
Win7 x64
--Oh, yes, let me also mention I'm also running MSE without a problem.
So...now that we have firmly established that the problem you are having is not related to your hardware, all that remains is software (assuming you do not have some kind of a hardware problem, which isn't likely, since your other games are running fine.)
The only thing I know of for a fact that will absolutely cause the frame-skipping you allude to is the Ubersampling setting in the advanced options. Turn that on and you are guaranteed a frame-skipping experience/extravaganza...
Have you manipulated your Witcher 2 user.ini file, I wonder? Because if you have, the only way to keep that from resetting every time you start the game is to write protect the file--which would mean, if that file is still write protected, that even when you try and turn off Ubersampling from the advanced options screen it won't actually ever turn off, if you have enabled it in the user.ini file. So make sure that your user.ini file is not write protected, and then turn off Ubersampling from advanced options.
If that isn't it, then *something* you are running is, for some reason, massively conflicting with this game. Run MSConfig and check your start-up programs and uncheck (disable) all of them that you don't recognize or that you know you don't need, and then reboot (changing these settings does nothing without a reboot, as you likely know.)
Also, you might want to temporarily disable your network/internet connection to make sure that your network controller is not polling every few seconds and interfering with the game--but I doubt this is the case because you claim to have this problem only when running this game.
What you describe, though, really does sound exactly what happens in my system when I enable Ubersampling. If none of the above is of any use to you, then my last suggestion is for you to first of all
set all of your system clocks back to default and reinstall the game (keeping your saves, of course, so you won't lose any forward progress.) Sometimes when software is installed via an overclocked cpu it may *seem* to properly install, but there were actually errors in the installation which crop up later when you try and run the software. I cannot emphasize this enough. People should realize that their cpus are not *validated* by either AMD or Intel to run 100% consistently at overclocked speeds. The manufacturer validates them *only* at their advertised MHz frequencies. Any higher than that, and all sorts of weird things can happen that don't seem to make sense. I know because it's happened to me--these days I do very little overclocking, if any.
Good luck!