If the contacts assembly is new, and pulls in, and carries enough current to get some SCR hum, then I would be looking to the few other things that may be 'direction of travel' related. I would also get the drive wheel off the floor as we test things, and insure that the the linkage to the accelerator pot was not part of the question. I had a Hyster last month that was a monotrol pedal and when the operator pushed on the reverse side of the pedal, the linkage was adjusted in such a way that the lever stopped pushing on the pot's arm, but only in rev. my point being, this unit may have some sort of linkage problem between the joystick and the pot or a bad pot.
I would check for this problem by reading the voltage on wire 29 on the ev100 card screw connection (tb 1)and making sure the voltage is the same in both directions of the joystick sweep.