Problem with the digital cluster PA 1987

Hi, I'm french and i write from Germany where i'm soldier.

I've bought a 1987' Park Avenue last month, here in Germany. that's not my first Buick in 2002, i'had bought a Reatta which i drove 1 year.

Here, it's a rare big car... It was imported new in Germany.

Then i've problem with the digital cluster, the tachometer is innacurate. I don't know why. I think the digital cluster is not the original but from 1988 or 1989, and it seems that is not the same connections.

What can I do ???

http://i4.photobucket.com/albums/y127/RICHIE1973/000_0068-1.jpg