I had this problem as well until I used the mdl export from a program called ultimate unwrap. It has a "smooth" setting that fixes it. It has been several months since I last used it and I've not set it up on my new machine, but Ultimate Unwrap exports very clean mdl. It has also been useful in exporting mdls with animations of all things...and imports and exports a good number of different formats.
http://www.unwrap3d.com/If ultimate unwrap isn't going to happen, I would try different export settings in your art package. What program are you using to make your models?
