There should be two wires on the sensor, one goes to the ECU and the other to the neutral light as well as the ECU. So only one will fire the light, between them they will tell the ECU which gear you're in for mapping purposes. Sometimes I wonder if you had a fault in this circuit if it would make the Ecu jump between maps and make the engine behave erratically. Those two wires should both be earth activated, check for continuity to earth on both while changing through the gears. If the sensor checks OK, then the wires or dash bulb will most likely be the problem.