So here's the weird issue i'm seeing with some degree of regularity:

Issue description:
2 balls are locked for multiball - via vault, thing or swamp, and they're both in the Swamp ready.
The game is doing it's "1-more-lock-for-multiball" music and associated light show, the power's flashing, everything is as normal...

You add another ball to the swamp, via the higher/side entrance.

The game warns about a swamp kickout but doesn't do it. Then hunts for the ball (there are now 3 balls in the swamp).

Eventually it fires one ball out, then another and then another - perhaps a second in between.

All 3 balls are now in play - the multiball music has stopped - because the games is no longer primed for Multiball. As soon as the 1st ball drains it tries to do the end of ball bonus but soon stops as the other 2 balls are registering hits and racking a score up. Normal-ish play continues.

I hope this is not normal behaviour!

Fault finding:
Running ROM L-5, the one before GOLD.
All swamp switches check out in test, no WPC error messages.
I've installed the swamp accelerator - just a piece of metal to speed the ball up in there. (http://www.leesparts.com/products/addams-swamp-accelerator.html)

Diagnosis?
Is my switch matrix up the spout?
Are there any tests i can do to whittle this down?
Even if 1 switch was faulty, why kick out all three balls? It must be detecting a successful kickout, actively feeding another ball from the swamp trough to the kickout, the repeating...

Help very much appreciated.

I may try to catch this happening on a short video.

Here's a video of the problem -

see about 25seconds in, volume up to hear the music change when the problem occurs.

UPDATE:

Replaced switches 71 & 73 in the trough and realigned all 4 switches in the assembly.

Also, slightly cheating, moved the knock-off assembly 2mm closer to the hole, so its poke action (snigger) goes that little bit further.

I tried to trigger the problem again late last night (as per my videos) but couldn't, so i set the Mrs on the game as she's more into TAF than I, but still nothing.

Either the problem is fixed, or much reduced to the point that it hasn't happened again since.

All i get now is the odd ball stuck in the trough which forces it into a ball search, so we give it big shove and it comes good. I think a ball is sticking on a switch or the knock-off isn't powerful enough on it's 1st poke.

I'll be replacing all the Power driver board Caps & BR's later this week too. That may help.

More detailed info posted to the UK Pinball Info forum: http://www.pinballinfo.com/community/threads/taf-odd-swamp-behaviour-kicks-all-3-balls-in-play-not-official-multiball.11714/

Cheers.

I also have that mod installed, unfortunately the problem still persisted.

Update: middle swamp switch was a little too close to the metal, thus occasionally stopping the ball passing cleanly over the middle switch. Now moved slightly away by a work colleagues who good at this stuff!

Not done the Caps yet.