I had the same thing happening to my STTNG when I bought it earlier this year. Turned out the drop target assembly was missing a small piece of plastic that the drop target sits on when it's up. Before I found out what it was the drop target would sit fine sometimes but others it would cycle constantly up, down, up, down, up, down, trying to catch on to the piece of plastic that was missing. Wish I could remember the part number for you but I can"t seem to find it.