Controller PCB randomly 'holding buttons down'

[Liveinabin]

Hallo all.
Here's an odd one. I have an arcade cab that used two MC Cthulhu PCBs for player 1 and 2s controls.
Recently, after upgrading the innards to a PC (cab used to run off a modded Xbox), I switched players 2s board for a Dual Strike (Windows couldn't handle two controllers with the same iD, kept switching P1 &2).

Now, the pinouts were a bit different but the Dual Strike was basically the same. So I hooked it up, checked the wiring and it works fine.

Well, it works fine for a random amount of time, after which the Dual Strike starts acting like all the punch buttons (to use fightstick parlance) are being held down. The buttons aren't physically sticking, the wiring looks good and I've tried putting the old Cthulhu temporarily back in there (and it worked OK).

So frustrating. If something's broke, I'd prefer it to at least be consistent.   Any idea what could be causing this?  I'm clueless.