@akka
This kind of display errors usually indicate faulty hardware, video ram being the main suspect.
Still, I've seen errors like that on one of my 5850s a while back and I got very sad. It turned out that one of AMD's CAPs had messed things up. Card still runs happily and can highly OC too.
In one more similar situation, my older 4850 started displaying artifacts and it turned out to be a faulty HDMI cable.
You can easily check if it's a cable issue, if you take a snapshot and go back and see it and scroll it around. If the artifacts are not there, then it's a cable issue. If the artifacts are still there, then there's a rendering problem caused either by faulty ram or bad software.
If the problem persists, you can try lowering the cards clocks in small steps, to see if it helps.