Since some time now I have noticed a horizontal stripe across the length of my screen about an inch from the top. When I look closer I see in fact two stripes, about half an inch from each other, but the second one is more difficult to spot.
I hooked up my laptop to my monitor and the stripes are still there. At least I know it's the monitor now. The refresh rate is 60 Hz so that's ok.
I reinstalled the Samsung SyncMaster 940BF monitor driver again and the display is much better now. The stripe is still there but less visible.