1994 Honda Accord Speedometer/Odometer problem

My speedometer often is stuck at 20 mph. When I first step on the gas it will try to fluctuate up a tad bit but it still sits around 20. As long as this is going on, the odometer does not run either. Every few days, the speedometer will simply shoot up to the proper speed and start working and the odometer follows and works also. But this is short lived as the speedometer soon is stuck at 20 again. Note: it never gets stuck at 0...only at 20??? What could be my problem and how could I perform diagnostic tests to pin point the problem???

Thank you,