after seeing the parts breakdown on this tiller control my thoughts are that if there are no codes being displayed but the unit just drops out of travel and the contactor drops out you could have a hardware problem in the tiller, maybe a broken wire in the harness in the handle arm at the base where it pivots or possibly where it comes out of the motor sleve and wraps around the motor before it goes over to the control panel area.
I would check for loose connectors, make sure they are completely plugged in and the retainer is snapped all the way in. I've found on many occasions where the plugs where together but not completely snapped and the way they are designed if it is not completely snapped it might not insert the pins together completely and cause bad connections.
I find it odd though that it is not giving an error code of any sort because these units are very sensitive and almost any interruption in any circuit will cause it to display some sort of code. Even if the batteries are low that usually makes the system throw codes because of low voltage situations in the circuits that run outside of the parameters the system is setup to run at.