I was having trouble with some horizontal streaks of incorrectly coloured pixels usually following large runs of black pixels. I eventually found out why: nVidia's DVI implementation is pants. The timing is out of spec at high clock rates. Fortunately on a card with two DVI connectors, the second connector uses an off-chip DVI encoder, which works fine. Voila, switch to the second DVI connector, and it works perfectly.
Took me a long time on google to find that out though.
