We are having a similiar problem although ours is with Stereo. We are using an FX 1000 card and in stereo we will see what appears to be the vertical retrace missing the swaping of the left and right buffers from time to time. This causes a flicker like effect. It occurs when we have alot of display activity in another window happening. I was interested in your report because to get optimum performance we are using custom mode lines as well. We have not been able to find a fix yet.