I have exactly the same problem with my Samsung 191T monitor and Geforce Ti4600 connected via DVI. I only see the problem with a DVI connection. Does everyone with this problem have a Samsung TFT?
As a workaround I tried setting the VertRefresh to 60. This did not improve matters until I also set the HorizSync to 64. Now it looks as it should (albeit at 60Hz... Of course still no flicker though being a TFT!).