Put the game in switch test mode.
Make sure all drop targets are raised and remove the ball from the game so no switches are closed.
Grab the manual and go to the switch test display numbers page (PDF page 12, paper manual page 17)

Activate each switch one at a time starting from switch 40 and work your way to switch 01. It's important you do it backwards because switch test mode will only report the lowest switch number closed. Raise the drop targets after you've lowered them before moving to the next switch. See if any switch closures report the wrong numbers.

Then repeat the process, but this time after lowering the drop targets, leave them lowered and move to the next switch. See if any switch closures report the wrong numbers.

If you can, post some clear high res pictures of the drop target switch stacks.

The phantom issues that are happening sound more like an issue with the #5 drop target diode as BigAl56 said - it might not be the diode itself, but rather that switch stack is improperly put together shorting the diode. This is why closeup pictures of the switch stacks will help.

Yes I suspect the spacer between the diode lugs is missing/in the wrong position effectively shorting the diode.
It could be a capacitor problem but we need the O.P to come back with more info.

Performing the switch test mode tests using the procedures mentioned in post #2 will reveal more.