[Have tried to plug-in the advice from the 'Efficiently Ask for Help' sticky, as best I could.]

Game acquired less than a year ago from a reputable auction house; game was in a private collection before that, supposedly well maintained. Observed issues did not manifest right away, and they unfortunately remain intermittent, rendering them harder to track down. Tech servicing from a good and capable source is ongoing -- a few visits to date, with the next one pending. Some issues have been fixed or at least improved, a couple others continue to stump all I've submitted them to. Yes, have the schematics. Stuff like soldering I leave to the tech.

The primary issue is that when a game is won -- either on points, on a Special, or on a Match -- the Credits wheel goes the wrong way and decrements one credit -- that is, goes backwards or deducts a game -- rather than adding one. Other times there is the "thunk" sound for a game won, but the Credits reel does not change at all. And some other times it does just what it should be doing. So far, this behavior has been mystifying. (But, have you ever seen this happen yourself ? If so, what did the cause turn out to be, assuming you ever uncovered it ?) A possibly secondary facet of this issue is that sometimes a first-level Replay score is ignored, but reaching Replay Level 2 (out of the 3) does indeed add one credit, as does RL3.

One other issue is that some Start button presses do not rack up a game: in order to queue up a 2-player game, 3 or 4 Start presses may be required. (ALL of these Start presses DO get deducted from the credits shown, as expected.) Again, this occurs intermittently. Related ? Unrelated ?

I think we've probably already exhausted the more apparent things the tech knew to look for. In searching other threads here, some things came up involving the coin mech connections. This game is on Free Play, and I don't even know if the coin mechs are functional. I've never tried to use them. Inside, the game looks pretty clean, with no obvious hacks. We do have one part on order -- which has to do with one of the spinners sometimes sticking ON noisily, but I'm thinking that is probably unrelated. If you'd like me to test something in particular, or photograph something, please let me know.

The game is very playable -- there are just these certain recurring glitches.

I just recalled a possible additional clue -- pertaining solely to the "3 or 4 credits to queue up a 2-player game" issue. Whenever that occurs, and a Start press is overlooked, a ball does not load into the shooter lane. It is clear that basic step is getting skipped. The ball does load for shooting on a subsequent Start press, when Player 2 is counted and ready.

Thanks for your contributions, currieddog & MarkG.
My tech was here this morning, for which I had the former's suggestions ready (plus some other hints pulled from other threads), but not yet the latter's. He checked a number of things, cleaned some contacts and plastic knobs, made other adjustments, and replaced a metal part that is no longer available anywhere as a fresh replacement and so had to be cannibalized from another game. There was a lesser, additional issue I did not mention, where the 100,000 points light came on very early (unearned) for Player 4. It looks like he fixed that one, and hopefully also the "extra Starts needed to queue up a 2-player game" thing. I'm guessing that currieddog's diagram was more related to that. In a variation of the old "Does that, EXCEPT when you take it into the shop", I never did manage to have the tech actually witness the "Credit Wheel Spins Backward" thing. There were numerous occasions when I could have made a video of it though -- but not predictably. While he was here, we scored numerous Replay or Special awards, and probably a couple Matches, too. Almost always the 'Thunk' sound, sometimes no movement of the Credit Wheel. (Which is pegged for the max. 25 Credits.) Not once did it go backwards though, while he was here. By the time he left, it seemed to be reliably increasing a credit. I'm thinking the coin mechs -- cited in other threads as a possible factor -- are not involved here.

I'll play this pin over the next week or two, and see how it goes. If the main problem recurs, I will look to MarkG's suggestions. Either way, I'll report back. And thanks again.

Thought I should update this, now that there appears to have been 85 % of a successful solution.
This had stumped my regular tech on past visits, despite his being fairly handy with the workings of EMs. I thought it might have been in this old thread I'd started, but it must have been elsewhere that someone posted a close-up screen shot of the credits wheel assembly, showing 3 vertical springs, with suspicions that one of them may have been at fault. It was the great EM guy Nico Volta (who I had the pleasure of meeting, during his cross-continent trip fixing EMs and instructing on the finer points thereof, a few years back) who pointed me in this direction. Well, here is the outcome.

First, there was some accumulated gunk in the credits wheel assembly, which had to be cleaned out. Then there was a lever that was not moving quite the way that it should. Nothing wrong with those vertical springs, apparently. But there is what must be *another sort of spring*, that looks like a medium strand of silvery wire wrapped round & round. (I may return to this post later and attach a fresh pic.) As a final step, the tech adjusted the tension on this winding strand. These changes seem to have corrected the problem -- almost. I am now getting single-increment addition to the credits upon a Special or a Match. With a game that is won, the credits are now increasing by 2 each time, rather than 1. And THAT is a HUGE improvement over jumping *backwards* by 8 or more credits. I'd even settle for this, but some further adjustment on that wound-spring may be called for, in order to get this exactly right.