1996 Infiniti G20 Speedometer/odometer and tach.

Trying to determine the source of two problems, and to determine if they're related.

Tachometer: Random readings, often pegged at 9000. Sometimes moves in relation to engine speed, but shows "0" at idle and only goes as high as 2000 at actual revs of about 3-4000.

Speedometer and Odometer: Occasionally fall to zero; seems to be triggered by hitting a bump, or, more often, when trip odometer is reset. If you manipulate the trip odometer button (e.G, pull on it or put sideways pressure on it, the speedo/odo pop back on.

Questions:
- Would replacing the instrument cluster fix the problem, or is the issue electrical (e.G, faulty speed sensor?)
- When I replace a speedometer, is it possible to keep the original odometer settings -- in other words, can the odometer from the old unit be moved to the new one?

Thanks much.