Do you use the E-S button on a regular basis?
That will eventually kill the relay on the motor board, because of all the extra recharge surges when you release the E-S.
It could also be the recharge limiting resistor(s) near the relay.
There is also a resistor or 2 in series with the relay as they usually have a 110V AC coil.
Please take a photo of your motor PCB so I know which version you have.
PM me if you want to contact me direct.