>I've seen this problem before, but I don't remember the solution.

I always get this problem, usually has to do with Modal form b though. Instead of actually tracking the cause down (I figured it will come and go anyways) I put:

oApp.RefreshAllForms()

in the Move() event of my modal forms, which just loops and does a quick refresh. If you include lots of code in your Refresh() events, might not be a good idea, especially if Windows is showing the form contents while dragging, that move is fireing quite a bit.