Hey BR, thanks for the reply.
I took it apart again today to take another look. It's very clean inside, mainly because this is about the 5th time I've had it apart.
The board looks great and the rubber pads have "normal" spring to them. Button press feels perfect.
I removed each switch and slightly bent out the springs on the turbo and A/B switches to make sure they are contacting the board properly. Then I gave everything a quick alcohol wipe, and re-assembled it, then re-tested. Everything worked normal except for Left on the d-pad, which has the same trouble I originally reported.
(Note that when this controller first started acting up, the issue was only with left on the d-pad, then everything was acting up, now it's back to just affecting left on the d-pad.)
To rule out a rubber pad issue, I took the controller apart again and rotated the d-pad rubber 180 degrees, then re-assembled. After that, I still had the same issue with Left only, so I know it's not a rubber issue.
I'm wondering if it could be an issue with the A/B switch, so I might try swapping that switch with one of the turbo/slow switches just temporarily.
The symptoms don't appear to suggest a wiring issue, and to my eyes the wiring looks secure. This controller did work perfectly for many months after the re-cable was done. I don't have any real way of determining if it's the wiring, since I can't easily switch it out.
The only thing that would leave as a possible issue would be the board itself.
Running out of ideas.