Yours is the kindof tech support call that no one wants to take. Basically, there's no clear-cut solution... you just have to test things and hope you nail it down. Here are some ideas:
1) Have you installed the latest patch for IWD? (available at
www.interplay.com)
2) Make sure you stop all unnecessary background programs. If you're using Windows 98, MSConfig is a particularly good way to stop them.
3) Try disabling hardware. Disable your soundcard first. I don't know if disabling your video card will do any good, because I don't think IWD will run on Standard VGA drivers (haven't tested this)
4) If you have the hard drive space, try doing a full install of the game. If you've already done this, try running a regular or minimum install instead, just to see if the performance changes any.
Not really much to say beyond that.