Here''s my problem... when I run my game as fast as it can, it looks great(smooth animation), which is at 100 fps(limited by the DDFlip and my monitor''s refresh), but when I try to slow it down to 50 or 30 fps it seems to be choppy. Any advice on how to take care of this?

I think what u need is some sort of timer function. A standard way of doing this is to measure the time used for drawinf each frame. When u need to update the sprite position u move em a distance in relation to the time it took to draw the frame.
Theres an article on this subject in the gamedev''s archives.