Fujitsu P27T-6P: Very bad picture on resolutions higher 1080p

I got a Fujitsu P27T-6P display. I connect it to my MBP 15" 2010 running lion via a Mini Displayport to Displayport-Cable. Its running perfectly fine on resolutions up to 1080p. However, on 2560x1440 I only get a very distorted, intermittend, snowy picture.

My Mac Mini 2010 has the very same problem. My girlfriend's MBP 13" 2009 (still running snow leopard) is working perfectly fine with the display.

I tried several resets, bought another cable, booted in Lion recovery and even from the Snow Leopard installation disk, the display problems remained.

I appreciate any ideas!