RM2K DISPLAY PROBLEM WITH 640X480 WINDOW?

I have a friend who recently got a new computer. He loves RM2K, but can't get it running 100% properly -- the animation is choppy and skips frames a lot when the program is running in a 640x480 window. I had this same problem once before, so I suggested he use fullscreen mode (using F4) or change to a 320x240 window (using F5). However, he finds fullscreen mode annoying and the 320x240 window way too small. His previous computer didn't have this problem displaying 640x480, so is there some way he could fix it? Any advice is greatly appreciated.