So i just recently received my 2 XFX 5870s. I bought them used on Ebay, they were apparently the best overclockers out of a lot of 30 cards. 950 Core, 1225 Mem @ 1.25V on air. Enough with the useless info, onto the problem.
I installed the cards properly with only one crossfire bride and the correct power connections. I have 3 monitors for eyefinity, but i'm just trying to get one working for now just for benchmarks sake.
Whenever I launch most any/all games the screen will flicker back and forth between a full screen picture and a smaller downsized picture. I've read up trying to find a solution but the best I came up with is to disable Catalyst AI, but that disables the use of multiple GPUs which completely defeats the purpose of crossfire. I also heard this is some kind of scaling issue.
It may be the overclock. Try going back to default 850/1200 and see if the problem continues. I am having a similar problem while overclocking/underclocking my 5870. Although my problems happen because of the idle clocks ( for mine is 400/1200 ), when overclocking/underclocking it makes my card idle at 157/300 which is too low for two monitors and causes the half screen ( looks like a window all the way across the screen ) to flicker.