Quoted from ian866:Love to see what you did with the motor housing earthing and where you tied it in. Is it possible it's an external component creating a spike and leaching into the motor circuits ?
The machine belongs to a customer and so a photo won't be forthcoming very soon. It's just a wire; one end attached to one of the screws on the ramp motor bracket, the other end soldered to the power ground on the node board. To be thorough I probably should have added another from the drum motor but didn't think of it at the time. Just throwing stuff at the wall to see what sticks. Plus the driver chip datasheet suggests grounding the motors to PCB ground.