96 jetta 2.0 manual trans Odometer/speedometer problem

does not work.. moved the cables and connectors and got it to work but
failed again. Cleaned the connectors on the tyranny. Does anyone know what
grounds need to be cleaned. If I drive the car and down shift or slow down
going down a hill. the speedometer will work for a little while.. I am
guessing it is a grounding problem but I am not ruling out the gears that
fit in the tyranny. Anyone fix a problem like this?