On my Firepower, I had the same issue. At the time the factory had assembled the leaf switches with one blade upside down (known problem w. Firepower), which wore thru the gold plating and made the switch intermittent. I bought new gold flashed contact rivets from Steve Young at PBR and installed them on the old blades. No more "F" problems.
Don C.

Steve Young (PBR) has the gold flashed contact rivets. I unsolder the switch stack, remove the switch go tot he bench. Now you have a choice. Flip the switch blade over and clean, or grind off the upset (smaller) side of the rivet, remove the old contact and replace. Cushion the face of the contact (hardwood w/ a layer of duct tape) and peen the rivet shank with a hammer.

Don C

Did the flipper rebuild include new flipper base plates? Once I replaced them the shot to the saucer has plenty of power, just aim properly and you will be there. Replacing the base plates is the only way to replace the coil stops.

Don C.

On the batteries, is there any green fuzz on the the batteries, their holder or nearby on the boards? Those look like carbon-zinc old school batteries, I can see rust, which is very typical on C-Zn batteries. Check that before sending out the board. No fuzz means no alkali corrosion.

Don C.

Yes, but before the dishwasher I would remove the battery holder (use an off-board holder) and scrub the area around where the holder was with vinegar and a toothbrush. Be generous, scrub a large area or the whole board. Rinse with isopropyl alcohol. Repeat those steps a couple times. This is actually the most important part of the cleaning, as the vinegar (an acid) neutralizes the corrosion from the alkaline in the batteries. Check that the board is still working properly. I don't think the dishwasher is necessary unless the board is covered in crud, but it's your choice.

Before starting anything, read the many Pinside threads on battery corrosion (often called battery acid, it's actually an alkaline, but 'acid' may be a good search term).

Don C.

At this point I will have to defer to the Pinsiders who are board experts.

Here is a thread on System 6: https://pinside.com/pinball/forum/topic/vids-guide-to-bulletproofing-williams-system-6#post-581542

Can somebody point towards a diagnostic guide?

Don C.