Not sure what the fault may be with having all the scoring problems in different areas, sounds more like a driver board switch problem, but If your problem with the drop targets in particular is related to the horseshoe wiper contacts, the best way I have found to ensure reliability is the double up each horseshoe wiper. Basically have another blank horseshoe (with the riveted contacts removed) behind it. This doubles the tension to make a good contact.

In fact this is how they were meant to come out of the factory according to the drawings which show two contacts present per drop target. Also the two horesehoes wipers have different part numbers as one is blank & the other a regular horseshoe.

I also have tried capacitors - they don't work on these Williams games, only Bally's. They mess thing up & add extra scoring as you said.

My Flash I am currently restoring is pretty much all original & had two wipers per drop installed. I also bought a Flash drop target bank a few years ago & that too had 2 wipers per drop.

Can be a bit tricky to remove the contacts.

These have obviously not been cleaned up & adjusted, but this is the idea.

P1020938 (resized).JPGP1020939 (resized).JPG

It sounds like you have everything just right to function correctly with the horshoe's adjustment, new PCB's & a bit of lube. That's how mine were when I did my Firepower retrofit with drop targets, I have never needed to adjust them once - that was done a few years ago. As you have done, the tension is just at the point where any more will slow the drop when dropping.

Ideally if you could swap over a known driver board to try so you can immediately eliminate that as you problem.

Looking at the schematic, IC 12 & 19 (7406) IC 15 & 16 (4049) & the switch matrix, the problem switches involved appear to be linked to row inputs through those IC's & the PIA IC 11. Just another possible fault with one of those IC's I guess.

I have been refurbishing a recently acquired Flash. It has a system 6 CPU & I noticed the ROMS were the "Yellow" version ROMS.
From what I have read their were 2 yellow versions for system 4 & a green version for system 6 (possibly the later yellow version with some system 6 games as mine had the later yellow ROM set)

It was documented that the only difference functionality wise between the yellow & green ROM set was the setting for background sound & a couple of other settings.

I tried a set of Green ROMS to compare to see if their was any difference with how the game plays & sounds. I did notice one difference with a sound between the two sets that had not been mentioned anywhere previously. It didn't matter how the settings were adjusted - their were two different sounds between the sets.

The first vid is with the Green ROM set - note the 10K is awarded with individual sounds, the second vid is with the yellow ROM set & the same 10k is awarded with a single sound (which I prefer)

Green ROM set is 1486 1 (3 Rom set)
Yellow ROM set is 0486 2 (3 Rom set)

I haven't used this decal, but I personally their is no way I would use this on that playfield, I don't think that playfield is that bad. Also, If it is removed later for playfield restoration it will take off paint.
If it was mostly down to wood in the area & looked really terrible, maybe an option.

If that playfield had the magic eraser treatment & nicely touched up & finished with a playfield protector it could come up great & fix all the problems with cupped inserts ect as well, If you are not familiar with what a "magic eraser" can do to improve a playfield it's worth reading up so you know it's an option . But not everyone is able to do or wants to do this.

If you can live with the wear for now, a protector is the best way to go to stop any futher playfield wear. It will look & play much better & you can review what you want to do later on, this way their are no regrets with applying something like that decal.

Here is just one thread on using Magic erasers.

https://pinside.com/pinball/forum/topic/cleaning-dirty-playfield-with-magic-eraser

Here is how one area of my Firepower playfield turned out after using a magic eraser, small touch up & Novus 2 polish. Most was engrained dirt nothing else but the eraser would remove.

P1020506 (resized).JPG

P1020566 (resized).JPG

Yeah, they are not sticky - just a pre-cut PET-G sheet that sits on the top of the playfield. They don't tend to move, if it does move a bit, a tiny bit of sticky tape in a couple of discreet spots is all needed to hold it firm, usually not needed though.

https://www.playfield-protectors.com/22/Playfield-Protectors

Protector review:

https://pinside.com/pinball/forum/topic/lexan-playfield-protector-install-and-review

I read a while ago CPR commenting on why they haven’t considered a Flash playfield. It was because the low value of the game would not warrant the outlay for the playfield. Maybe not the case nowadays.

That playfield should come up looking fantastic after restoration - the cracking looks like it's mostly going to be in the old lacquer which would get sanded off. Interesting how cupped those arrow inserts are for a NOS - goes to show how it's not really lamp heat but moreover the weak structure of the inserts that causes severe cupping.

I bought a NOS set of Flash playfield plastics from that seller - very happy with them. Great seller.

I believe Flash did come out of the factory with blinking bulbs behind the lightning (only)
I have done that too & like it with them.

A lot of people are fond of Flash in Australia, you will have to pay at least double to what you would pay in the States for one in Australia - I think a few will make their way here.

Their is a ROM upgrade that exists that adds lane change to Flash - I thought it would be worth trying but I could not get the ROM to work for some reason.

https://pinside.com/pinball/forum/topic/does-williams-flash-have-lane-change#post-5492810

This thread is worth reading - same issue as yours & with a Flash too. Resolved with CPU connectors being the cause of the issue in this case.

https://pinside.com/pinball/forum/topic/williams-system-4-5-volt-low

This too...

https://pinside.com/pinball/forum/topic/gorgar-resets-wboth-flippers