It can be a number of things from the info we have so far.
The motor is used and can have internal issues, the wiring may be in poor condition
and lastly the dash switch may be on its' last legs.
I would start by testing the motor all on its' own using the shop manual.
Once that checks out , then test the dash switch. If both are good then
check the linkage for any worn or broken parts . Lastly check all wires for
poor contacts and /or breakage.Most important make sure you have good clean contacts
for the grounds.
Jules