Quoted from Jakers:I had this same problem come up on mine not too long ago. The solder on the receiver opto had broke and was touching the metal rail. I soldered the wire back on and put a piece of electrical tape over the back of the opto board. Been working 100% ever since.
I believe this was an issue with Stern a year or so ago(or a fellow pinsider), that it was being grounded on the rail.