i am guessing here, but....
if it is cable driven, there are usually 3 parts, the worm drive, the cable and the rev counter.
remove cable from rev counter and see if it turns smoothly when engine is running. if not its the cable or the drive.
follow cable back to block and locate drive, remove inspect for damage or wear.
if all is ok its probably the rev counter faulty.
if its electronic, similar but rev counter, wire, sensor. but youre on your own there!