It might be that the main circuit board in the cluster is defective. Dealer and usual sources ask a hell of money for that.
Ask here on the board if someone has a used cluster, can also be from an E32, as long as the color on the back of the cluster is the same. There are grey, white and blue color backs. All are from different production years.
More details on the page of Johan and Sean under 299.000 km.
That is where the mileage counter freezes on very old models.
Their bewiste you can find on my tech help page below.
You can also try here or on Ebay.
http://stores.ebay.com/BMW-Part-Shop...QQftidZ2QQtZkm