I've borrowed the shop manual from my dealer, and the only thing it says about the hand throttle is it detects the amount of movement in the hand throttle lever and sends this information to the ECU which then adjusts the fuel injection amount and timing accordingly. Two sensors are integrated into one unit. Sensor 1 detects the main displacement of the lever and sensor 2 detects any faulty condition. If the difference between the main displacement signal and auxiliary displacement signal is over the specified level, the ECU sends a fault signal. If it's a fault, an error is displayed on the instrument cluster.
Have you tried taking off the plastic cover to see what the sensor is doing? It may physically be moving when you adjust the throttle lever....like it's loose or something and is turning when it should be stationary. If it moves or turns with the lever, it's not going to work right. You fiddling with it may cause it to eventually "catch" and it works afterwards.
Guess I should mention the sensor is attached directly to the bottom of the throttle lever.