I have a FOER 9/2.5 Lansing Bagnall serial no. 80084911. When I go forward or reverse and get to a certain speed the forward or reverse contact starts to kick out and in repeatedly until I ease off the pedal. Voltage is at about 67 volts when this happens but if I go slow and keep the voltage at 68 and then lift mast the voltage drops to 65 but the contact stay engaged. What could the problem be and how do I fix it.