Brakes sometimes work sometimes don't

What kind of controller? My first reaction would be the controller isn't working correctly. Maybe you can borrow someone's controller for a couple of hours to see if that makes a difference. Other obvious troubleshooting step is to check the truck end of the plug to see if current flows when you step on the brakes; although having said "barely moving" it works, that is probably not going to provide a definitive answer.

Sounds like you have a loose or broken wire . Check the wires where they exit the brakes. Clean connections.

The first thing I did with my service trailers/rv trailers and boat trailers when I purchased them used or new is start at the front of the trailer and work to the back replacing all clamshell connectors. I stripped the insulation off a good bit and twisted the wires together...then soldered them. I then wrapped them good with a tough electrical tape...then shrink wrap them. This resulted in full selected amps to the brakes.

I would start by trying to find any kind of instructions on the controller. It's possible there is a troubleshooting section that mentions the flashing red light.

Does it flash red when rolling down the road, or only when you step on the brake pedal at speed?

Brake wires have been known to short to the axle tubes where they run through the axles. Be sure to check that the insulation is good there.

I found a broken wire inside the drum not too long ago. If nothing else fixes it, you might want to take a peek inside.

Had the same problem as you, only no lights of any kind. Slow down brakes would barely stop, checked all my grounds and connections, still won't work right, so I took my trailer to a shop that specializes with trailer brakes, axles, and electrical. Find out the last person that worked on my brakes four years prior, pumped to much grease in the fittings and blew out all four seals. Had to clean everything up. Thought I might have needed new brake shoes, but original ones still in good shape. Work good now, might want to look into that.

Once the brake shoes are saturated with grease, they will likely retain some so the proper fix is to replace. Once again the useless grease fitting strikes and the proper fix for that is to smash it with a hammer so it never gets used again.