First, find out what maybe the cause. Unscrew the cable end from the back of the speedometter housing. If the needle movement stops, its the cable and it needs attention. Carefully remove the cable. Clean it with solvent. I use liquid graphite and lightly coat the cable and reinsert in housing. You will have to probably twist the cable to get it to seat in the gear housing. Add some more liquid graphite into the housing before screwing it back on to the speedometer housing.
If it is not the cable, then Tinker's suggestion will be the next step.