The speedometer on my '87 240 drops to 0 while driving. It usually jumps back to the correct speed if I tap the cluster or hit the top of the dash. Sometimes it takes a few hits. Problem used to be occasional, now it is happens about every few miles. Every now and then the speedo reads low but not zero - for example the car is going 70 but the speedo reads 50. Hitting the dash or cluster generally makes it return to normal in this case too.

Most of the posts I've found say to check the sending unit wiring for speedo problems. However, since hitting the dash fixes it I suspect it is wiring in the dash, or I suppose it could be internal to the speedometer.

Where to check?