desktop icons appearing, disappearing

I'm running a 2.4 GHz Intel Core 2 Duo large flatscreen iMac OS 10.5.5. Recently 4 desktop icons (for 4 jpeg files and one folders) have occasionally gone partially obscured until I click on them, when they pop into full view. Alternately, when I log off or shut down, those same icons sometimes remain partially visible after everything else has shut down and vanished except for the desktop image and the spinning shutdown wheel. No operations seem to be affected. The affected icons are in the lower half of the screen within three inches of the right edge. I have icons scattered in a 3-inch strip along the top and a 4-inch strip along the right edge. Only these 5 icons are affected but I there are no others right next to them. I ran Onyx, verified the drive, repaired permissions, etc, and am still seeing the same problems. Any ideas what's happening? Anything to worry about? Thanks!