You are looking in the right area, there is probably a loose connection or cracked solder joint on one of the opto boards and the vibrations of ejecting a ball are causing an break in connection. I would do a close examination of both opto boards and attaching wires.

What about the 7th opto?

Ok, I understand your problem a little clearer now; the game pre-stages 3 balls, one under each of the 3 VUKs. If you have a diverter issue, the game will endlessly cycle balls trying to accomplish this. This is not what you are seeing though, you are seeing it try once and stop; typically, this means you have a bad opto in your VUK, the game sees no ball staged, tries to stage it and sees that it ends up at the 2nd lock. Best thing for you to do here is to remove all your balls and look at what optos read closed, it should be all of them; whichever isnt registering needs to be fixed.

This is what you should see in your switch matrix with all balls removed, the green boxes represent closed switches.

STTNG Switch Matrix-no balls (resized).jpg

Check more than the opto. Check the IDC cables connecting to the opto boards.
On my STNG; vibration of flippers and/or trough coils was causing intermittent optos.
This could be seen by wiggling the trough board wires going into connectors at the trough boards.. this would cause switch transitions in the test.

Good idea for the trough boards;however there are no IDC connectors connecting the VUK, optical sensors, these have always been molex type connectors.

Picture or VUK optical wire repairs and VUK's during full restore:

1130 (resized).JPG

1131 (resized).JPG

The photo transistor hardly ever fails, normally you see the the transmitter go. Also keep in mint these optos are all controlled by the 16 opto board under the play field, there are IDC connectors there and a bad connection to that board can also be a problem just as Zitt was eluding to.

Exactly. The connections to the opto trough boards are indeed IDC (insulation displacement connectors)... Infact most board to harness connections are IDC.
Specifically on my opto boards the +12V power wires were failing due to age. So I replaced that connector with a crimp on style connector and final Opto problems disappeared.

Yes they are; however, there are zero indications of a trough board malfunction in this thread. I only see a VUK opto issue. I may be missing something though as I don't think the game is actually ejecting 2 balls into the shooter lane at the same time, if it is, then I'm way off.

Were all the LEDs on your old board working before you switched it out?

I'm having the same issue but all the LEDs on mine are ok and the receivers are also.