Quoted from CadillacMusic:Could be a bunch of things. My #1 thought is that the 6 ball trough opto is acting up, because those always fail. You can get a new set of those for like $80.
I would not recommend replacements unless you have no ability to solder, or the ones you have are fubar'd beyond any repair. If you do, please give me the old ones if you will!
The new ones are all SMD and thus most of us can't repair those, and those optos seem to fail even more frequently then the through hole ones do. I have never had to deal with a repaired opto board again in my time, but people I know that bought the SMD ones seem to have an issue with them 2-3 years down the road.
My advise is that when there is any possible trough opto issue, pull both boards, break the piggy bank for the $5 in parts and install all new receivers and emitters. My experience is that if you have one intermittent opto and fix it alone, you will soon have another, and another.