Mike, I just fixed my 90 740 yesterday, with the same problem- started out acting erractic for a while, then eventually stopped working altogether; are you losing the odometer also? Mines turned out to be one of the two wires going to the speedo sensor on the back of the rear axle had become worn and was making intermittent connection. According to the 700 FAQ's, this is a common problem. If I drove over bumps or rough roads, the speedo would really act up, so I suspected it had to be something related to the vibration of the axle. It was real easy to fix- took all of 20 minutes to splice and fix the wire, and everything's back to normal. Give that sensor wire a check before looking at your speedo.
MikeN