Just for completeness (I am not familiar with the csmio) as this thread is long is the 24V ps -ve connected to the -ve on the controller board.

I would be very surprised if it turns out to be the switches although I favour npn type.
As has been said I don't think the cable length should be an issue.

Have you tried a small capacitor at the machine end of the ps supply cable across the +ve and -ve.