Problems iiyama prolite e435s + radeon x600 pro


I have just purchased a new lcd screen iiyama e435s (10 ms) and on my x600pr video card it works fine for about 2 minutes and then the screen starts jumping up and down.
I have tried connecting via DVI and norma DSUB. I have this problem on 1024768, 12801024 in 60Hz modes.
On my other pc the screen works fine (Nvidia Ti4200). Am i overlooking sumting? Can this be related to any setting of the video card?

Hmmm, it seems the screen works fine for a certain period of time (until it warms up i guess) and then it start jumping up and down on my other pc too. I think it may be defective. When i let it cool down it works fine again for a certain period. When i dont let it cool down at reboot pc it keeps on jumping up and down…