Hey Garry, I'd check the 4 optos on your drop target assembly. Go into switch test and see if it's registering a closed switch at one of those positions. If so, the CPU is reading the drop target is still down via the switch matrix, even though the actual target is up. Most Williams CPUs will only give about 5 pulses to correct something before stopping. So, it's trying to correct the switch error it supposedly sees and just giving up after the 5 tries.
Potential solution could include reseating the J1 connector that feeds the opto board and visually check if any wires are loose or missing from it, or you've got a misaligned or faulty opto on the board.