Hi,
Is there any choke on the motor cable, Check suppression on contactor coils fit some if not, carry out insulation test on all motors and cables even battery cables. Has it a beacon on disconnect if it has, check any relays coils one might have a dodgy coil, Check mast switch is correctly adjusted is they any play in mast rollers, is they any plastic warp around wheels causing static electricity building up, just look for anything that might sent interference back to the zapi controller that about all I can think of,

Pete