Curious. If it goes off next time, where exactly would you test the 12v circuit tester. Like at which point would give you possibly where the power is going off or cutting out from battery > controller.
Im not the best at electric i hold my hand up, can just do the basics from experience with wfp over the years.
Ok, here's how to do it. Get yourself a tester/lamp like this with a sharp point on it so you can push it into a wire to test. They're not much money and they're a handy tool to have save you so much time eliminating faults.

You've got two wires going into your controller that we're interested in. The red positive one and the black negative one:

Put your test clamp on the negative battery terminal and probe the red wire as close as you can to the controller. If you get no light then you have no power, so work backwards to wards the battery and probe the red wire after the first connection you come to , carrying on until you get power. That should eliminate which joint/connector is giving a problem. If you do get power and a light that means obviously power is getting to it. So now to test the black/negative wire:
Take the fuse out so the controller is not live. Put your clamp on the positive battery terminal. Probe the black wire close to the controller. If you get no light, then your negative wire is broken somewhere along, so repeat the proccess after each connection intil you get a light and eliminate which connection is faulty. If you do get a light straightaway near the controller that means your wiring is fine and your problem lies in the controller itself.