It happens to me fairly often. Switching back and forth between views (Hidden Lines removed, Hidden Lines visible, etc.) almost always brings it back.
Yep I get that bug too. I great graphics card will help fix that. In the mean time make sure you're not in lighweight mode, that too contributes to such happenings. I usually either ctl-Q. If that doesn't fix it then I just Hide the view via the feature tree manager then re-show it and that tends to help. Then if worse comes to worse I just close the drawing and open it again.Thanks, Daniel