If it reading 64,200 rpm during start you are picking up intererence from the glow plug circuit. This will drive the ECU nuts and the turbine will most likely not start. Seperate the rpm sensor lead and the thermocouple lead from everything else as much as possible. Twist the glow wires around each other and the starter wires around each other. Make sure the glow plug lead is well grounded. If you have it under a case screw or on the mount put it back under the glow plug. Since it had been starting ok to this point I would suspect a poor glow plug ground. Also make sure the glow filiment isn't touching the inside of the plug.