I've been following this discussion since I was having a similar problem with the pop-bumper or thumper bumper on my game.

What I found which fixed the problem:
The solder joints on the upper-right flipper coil were defective.
The diodes (2) had both come free from the circuit and were making intermittent vibratory contact.
This was the result of a previous "repair" done poorly.
Easy fix. I'm hoping this is where your intermittent problem could lie.
Maybe take a look at these connections at all of your coils? Inspect for dull/sloppy/cracked joints.

The reason I write this is because problems rarely "fix" themselves. They just get worse over time and turn into a more serious repair issue down the line.

If it is constantly high and never goes low there is a problem on that line. It should be mostly high with low spikes (when selected). If you can trace continuity all the way back to the PIA on the MPU and there is no breaks that leaves the PIA or some kind of short. Could still be a lack of continuity, just the pressure from the your DMM leads is making the connection.

I wouldnt think a PIA fault could be intermittant, but maybe... also shorts are rarely intermittant as well. Try swapping the two PIAs at u10 and u11. Maybe the PIA socket is flakey.

You now know where to thorougly check though.

I find myself still chasing my own problem too. The thumper still occasionally gives a hick-up in relation to the flipper buttons...........strange that our problems are similar.

This past weekend - the thumper quit working entirely.
After an afternoon studying the board and checking the resistors, transistors, etc.....
I found the problem in the "ground" or return line which is common to a number of items. The way I found it was by physically chasing the "suspect" wire throughout the wiring harness to the point of fault. Took a lot less time than it sounds like.

Perhaps you have a solder blob shorting the wiring harness?? This thought is what led me to follow the wiring harness carefully. I was not being cautious while making other repairs under the playfield and had a splat or two.

The targets on my game share a common ground with the coils - flippers, gate, drop-target, etc.
Ground wiring had come loose and the wires had separated thereby opening the circuit. Why all of these items were not effected is beyond me. Some things still worked??
I'm considering just re-doing the entire string in sections as time allows. Grab the schematic and go point-to-point re-soldering connections and replacing the small caps and diodes at each target, coil, switch, etc. Simple work - cheap cost - time well spent IMO.

Edited to say: The thumper still gives the occasional hick-up during game play.
I will continue to follow this discussion to see if you turn anything up.

Fanatic,

SED6's issue is on the encoded side of the solenoid signals. Sounds like your issues was past the decoder.

The the bumpers firing with slings is normal to a point in a classic bally and unavoidable. Things to help isolate flipper noise getting into the switch matrix is to have clean end of stroke and flipper button contacts. The bigger the spark, the more noise there seams to be. Sometimes the ceramic disk capacitors on the switch matrix switch can go bad as well.

Make sure game is grounded properly. The side rails and all metal should be connected to the earth ground.

Switches run at 5v and low current. I doubt you could feel it. Solenoid voltage will make you jump. Display high voltage will light you up.

Best practice is to not adjust switches with power on.

Yes, you're correct. Decoder was fine in my case.
Excellent suggestion with checking the EOS switches - very good observation.
I was wondering if the "back EMF" from the coils could make it onto the control signal cct. You've confirmed my suspicion as valid. Thanks big time.

SED6 - Please keep updating as you make further progress.