Are these the ones which have trucks with the gear cases made of metal halves? There are no electrical wipers on the wheels or on the contact areas of the truck to chassis? If yes, did you clean and burnish the white metal areas that electrically connect the trucks to the chassis? White metal oxidizes and just simple cleaning will not remove oxidation. Also did you clean and re-lubed the inboard axle bearing areas?
I think you may be confusing the PAs with early Fs. All the PAs were made with the "modern" Kato chassis (see Spookshow review, here-
http://www.spookshow.net/loco/katopa.html ). Some of my PAs date from very early on, and the chassis and trucks are as shown in Spookshow's review.
Of course, there are the units Kato produced for Con-Cor in earlier decades. And, for that matter, I've heard stories of auction sellers swapping Life Like models into Kato boxes.... but I digress...
On the OP's question- if you haven't already done so, double check that all the brass strips are properly seated, connecting and corrosion free. Also, one issue I've had in the past, the tabss that fold over the circuit board sometimes do not make good contact, and/or, the little locking clip (the plastic bit that fits into the center of the board that holds the tabs down) is not properly seated, causing intermittent problems.
