Quoted from Purpledrilmonkey:This sounds like a bad opto to me. I'm no expert on how this whole system works on ST but I believe the opto just behind the drop, but in front of the magnet, controls all of this mess.
This ^^^. I had the same problem and had to replace the left opto right behind the drop target. Unfortunately it was a huge pain to get to. Test it in the diagnostics to make sure.