R40 2681 - periodic black lines on display

[MinisterK]

Hi everybody.
I own a Thinkpad R40 2681 HTG, originally equipped with a 2GHz P4 M, 768 MB RAM, Radeon mobility 7500 with 32 MB Video RAM, 1400x1050 TFT.

Now I had to change the mainboard because of a broken solder connection below the GPU.

On eBay, I bought a planar with a different FRU than my former one, but originally for R40 2681 Thinkpads.
There is a different GPU on it: a Radeon mobility 7000 M6-C16H with only 16 megs of Video RAM.

My problem:
Every 2nd column of pixels on the internal TFT is black (like an interlaced mode, but vertically), resulting in an nearly unreadable picture in higher resolutions. That occurs from the first moment the unit is switched on (IBM splash, booting...) and in every resolution.



There is no problem with the picture on an external connected TFT.

Is there anything I can do? Is that a known problem?