Hello, according to the manual, the 54C3 error looks like:
54C3 | MOSFET s/c M1>B+ MOSFET | IGBT s/c detection on M1 top devices
The IGBTs are part of the electric power switching system for the main drive. M1 indicates one of the phase wires going to the motor. Since it's intermittent, I suspect dirt, corrosion, or contamination on the electrical contacts. I'd think about cleaning the contacts. I'd suggest using personal protective equipment (nitrile gloves, maybe leather gloves on top of the nitrile gloves), safety glasses at a minimum. Unplug the charger, make sure the key is off and removed. Then, I'd suggest removing the seat, and using a DRY toothbrush (or similar plastic brush) and brush all the contacts on or near the motor controller module (and the main contactor/relay/solenoid). (This is a black module fastened near the middle batteries). I'd also think about using a compressed air gun (don't let the tip get near the batteries or any of the contacts!) or a spray duster and spray around the contacts at the motor controller. Then, I'd go to the motor itself, and look at where the wires are attached--see if you can see crud, dirt, corrosion, etc. and clean around the motor and where the power wires connect to the motor.
Good luck, and please let us know how things go!