Dave
Only thing I can think of is to check all the connections going into the board and also the pig tails. Some of these shells are tight and leave little room for play. Reinstalling the shell means making sure all the wiring is not catching on or being pinched by the shell. I know its a pain in the ####.
Funny I am having a similar issue with one of My 3rd Rail FT A units. Shell off Full TMCC control. Shell on Starts up, Directional control, but once starts moving no control. In conventional mode operates only in forward no reverse. Take the shell back off full control again. Must be a Saturday thing.
Doug