The very first thing you need to do is go to switch test mode after the switches start playing up. It will tell you if the game is sensing those switches as stuck closed, i.e. if there's a short on that "I5" switch return line.

The start button is also on that "I5" switch return line. If after those playfield switches start failing, you are still able to start games and add players as normal, the problem is not related to U10.

This doesn't test the continuity of the brown-yellow wire through the J2 connector though.

You did this when the game was in failure mode right?

With the machine OFF, can you check for zero ohms resistance from the right leg of R52 to the brown-yellow wire soldered on one of those failing playfield switches?

Those 3 switches you're having problems with each has a capacitor on them; have you tried clipping one leg of each of those capacitors to rule them out?

Does this MPU board have previous battery corrosion? If yes, post a clear high res picture of it.

Thanks for the pics. Looks to me like the corrosion might still be active on the board.

Yes, but in the mean time, can you take some resistance measurements on the MPU board hooked up in the game with the machine OFF:
Set your multi-meter to ohms "Ω" resistance mode (200K scale if your meter isn't auto ranging).
Black meter probe on ground, red meter probe on the left leg of resistors R62 through to R69 (eight resistors located next to connector J3 bottom right of the MPU board).
Report back the eight resistance readings.

Can you remember if the Start button also stops responding when the problem occurs? It's sitting on the same row in the switch matrix as the other switches that play up.

Those resistance readings are all good and since the start button continues to work, the fault itself isn't the MPU board.
Have you had a chance to get the game into switch test mode when the issue occurs?

Your issue is likely one of the capacitors across one of those switches is shorting (have you tried clipping them yet as suggested)?
Or you need to reterminate the J2 connector at the MPU board particularly pin 13.

It's very possible. The switch return signal you're having problems with is also used for selecting numbers being displayed. We've seen this problem before with other pinsiders where displays behave weirdly with start button shorts to ground.
Also check the diode on that Start button switch isn't touching the coin door lock lever or coin door itself.

If you can't see any issue at the switch and the display issues return, pull the J3 (cabinet switch harness) connector off the MPU board while the machine is on and see if the displays return to normal.

When those switches stop working, what happens when you go to switch test mode and press them? Is there no change (continues to show '0' at the Ball in Play display) or are you getting the wrong number (per their ID in the manual) shown for them?

Unless you shorted something when cutting the capacitors, there may be some bad diode in the switch matrix.

Are either wires that go to the start button switch being pinched around the coin door hinge?

Re-terminate that J2 connector at the MPU board.

If the three switches aren't working, remove the playfield glass and open the back box.
Press and hold the "C" target. With your other hand wiggle the J2 connector at the MPU board. Does the switch start scoring as you wiggle J2 especially the brown-yellow wire at pin 13?

Grab a piece of wire and strip 1cm from each end.
Insert one end into the J2 connector at pin 13 (where the brown-yellow wire is)
Insert the other end into the J3 connector at pin 14 (where the blue-white wire is)

Both these locations are the "I5" switch return signal at the MPU board (J2 for playfield, J3 for coin door)

If the jumper wire is making good contact in each pin/terminal and the issue goes away, you know the problem is the connector at J2.

There's no doubt this MPU board needs to have the corrosion properly dealt with going forward.

Scmitchell from post #45 grab that piece of wire and insert one end into the J2 MPU connector at pin 13 (leave it there) and briefly insert the other end into J2 pin 4. The "C" target should activate (maybe other switches too).
Next if you briefly insert the wire into J2 pin 5 the bottom pop bumper should activate, if you insert into pin 3 the dragons den target should activate.

Next time those playfield switches stop working do this jumper wire check again.
If nothing happens, then remove the wire from J2 pin 13 and insert it into J3 pin 14 with the other end briefly inserted into J2 pin 3, 4 then 5 and tell us if there's any response.