OpenSuse 11.1 screen dimms while scrolling


while scrolling with the File Browser or with Firefox on a wide window (say 1000pix) the screen suddendly dimms slightly (about 30-40% intensity) and comes back when scrolling on to another area. If I reduce the window width considerably the effect goes away. I am using a large display of 1600x1200 and nVidia GeForce 7300GT. The effect does not appear under Windows XP nor under SuSE Linux 10.0.