This is what the IIgs looks like connected via the OSSC, note the beat pattern on the blue desktop background.
Direct link to image as the forum is resizing the above: http://i820.photobucket.com/albums/zz122/njroadfan/DVI2PCIe/IIgs%20640%20OSSC_zps3objcane.png~original
Playing with the advanced timing settings just increases or decreases the frequency of the beat pattern on the desktop background and never completely eliminates it. The Apple IIgs is connected to the OSSC via the VGA port (AV3).
This is what it is supposed to look like via a direct capture from my capture card (an emulator will produce similar results). The background consists of 1 pixel wide blue and white vertical lines to create a dithered periwinkle color.