I had something similar in my 91 745T.
I did all the suggestions, but nothing fixed the speedo, but the intermittent ABS light was fixed by fixing the wiring at the sensor on the rear axle.

My issue ended up being intermittent speedo, odo and ABS was good all the time.
I bought a 96 dash panel and nothing in the panel was interchangable with the 91, so that was a waste of $. Anyone need one for cheap?

I tried everything I could to get the speedo to work consistently and nothing really fixed it. There's no circuit board, or oil, as someone mentioned above, that you can fix.

I bought a 91 cluster, put that in and the speedo was fine, but other stuff was intermittent. Took the speedo out of the new cluster, put it in my old cluster and it's fixed.

Long story short, you're going to end up with a new speedo - watch eBay. I never found one. Paid $75 for the cluster that fixed mine.

mike