This is actually a known issue. It has to do with the idle clocks/voltages dropping extra low when the Catalyst Control Center Overdrive is enabled. If you notice, when you are NOT overclocked, your idle core clock will probably be 400MHz or so, but when you start OCing, that clock will drop to like 157MHz. The idle memory clock is 1200 I think, but when OCed that also drops. I believe this is what causes the flickering you see. (My 5750's do the same thing). I think you can fix it with some messing with the BIOS and raising the idle clocks/voltages, but I've not tried so I can't tell you exactly how. A little googling on that and you should be able to find the info you need.
rolli59- I don't think thats true, just doesn't like the super-low idle clocks. Some cards may not have the problem because they can handle it, but it seems to be quite a common problem, even when the cards overclock quite well.