annoying border around frame (redisplay problem?)

Trying with several versions from the 21.5 and 21.4 trees, I have a fairly annoying display problem. A thin white/gray border around the frame, behaving extremely inconsistently. Depending on what I do it will surround the frame or not. A particularly ugly example is here: <http://khaberz.net/Images/xemacs.jpg> If it is there, C-l will consistently remove it until I do something within the frame when it will reappear. So I assume there is some sort of a redisplay problem. Is there some sort of workaround? A face whose color I could change to match my default background? Thanks, Kai