39E is that the primary engine speed sensor or wire has a problem. The primary speed sensor is in the injection pump, Check the 6 pin connector to the injection pump, look at the wire ends to see if there ok on both sides, connector wires B and A. Check with a ohm meter between B and A for 600 to 1500 ohms to the pump.
Could look at the speed sensor on the engine, take the connector off and look at it, this would be a 44E code.
Don't understand why going down the road will do this.
Had a 8300 tractor that the primary sensor was bad and the code was always on, the sensor in the pump was bad and replaced the injection pump.