Windows XP?
I have direct draw and I still encounter this problem fairly often... I change the resolution or put it into Windowed mode, and sometimes it helps... but I've never really figured out how to get rid of it completely.
http://forums.delphiforums.com/SWWISA/start
the aforementioned forum is probably where the best and brightest minds in Red Baron 3d land frequent... so you might find some help over there... if you haven't already done so.