The only issues I've had with my Stab HH90 motor were all cabling related. I had troubles where it would just return to center. I bought a compression tool and put compression ends on all of my cable runs and that fixed things up. If at all possible, can you get a short (25 ft?) RG6 coax cable, take the receiver and tv outside and see if it reacts fine with a different RG6 coax cable attached?
Also.. When I was having Stab troubles, I noticed that the dish would only move when I selected Horizontal transponders, which move the dish at 18V instead of 13V. Can you try delete/recreate a satellite and only manually put in a Horizontal transponder to see if that helps? That's what I noticed when I had bad cable ends.