Junk wiring on the trailer.
Typical spot for intermittent wiring faults is the wires that run INSIDE the axle tubes. Those wires are loose inside the tube which leads to chaffed insulation which leads to intermittent shorts in the wiring.
The moving wires also can eventually break leading to loss of brakes..
Best bet is to scrap the existing brake wiring on the trailer and run NEW wire and wire tie the wires to the OUTSIDE of the axle tubes.
Replacing the wire is the easiest and fastest way to fix intermittent issues.
You can also use that as a upgrade route, move up a couple of wire gauge sizes and pickup better braking..
