I guarantee the MOSFETs on the driver board will be shorted - a VERY, VERY VERY common problem with this machine.
Replace them ALL with logic level driven FETs, nothing else will work correctly.
There are a few mods you can make to help the circuit to be a little more reliable but unless you are prepared to relocate the driver board to within 6 inches of the MPU in the backbox and extend the heavy wires to the coils from the relocated driver board you are likely to always have issues with this part of the circuit.
The general thinking is that ramdom spikes get into the control wires of the driver board and cause it to lock up - relocating it so it is close to the source of the control lines minimises this interference issue.
Basically it is a design fault, built in from the factory.
