Speedo is still erratic:

I own a 1992 740 wagon with about 180,000 miles on it and the speedometer has been acting up. When driving at about 20 mph the speedometer will start to jump up to about 50 mph. As I increase my speed from 20 mph the speedometer slowly increases to about 60 mph. Once I’m actually traveling at 60 mph or faster, it seems to be accurate. It has never actually stopped working, it just reads too high. It usually works fine until I’ve been driving for about five minutes or so. The odometer works and I think it’s measuring the miles correctly for the actual speed I’m driving – not the speed the speedometer reads. I ran new wires from the connecter at the pick-up unit on the differential to where the wires enter the car and also removed the cluster and the speedometer unit and tightened all screws. The green circuit board on the back of the cluster doesn’t appear to have any discolorations. I also checked the ground located to the left of the driver's feet. The problem is still the same. I've also noted that sometimes right when I start the car the speedo jumps up to about 40 or 50 mph. Anyone have any suggestions or know of other grounds I can check?

Thanks for your help,
Bolbo