It's also possible that you had a loose connection which you've solved. I hope that's it; however...

I would think that it is POSSIBLE that one of the cables has interior corrosion that isn't at the surface of the terminals. You're manipulating the cables may have temporarily solved the problem SOMEWHAT.

I have a problem with my inverter on a prior boat (a couple of decades ago). The inverter and the voltage meters showed good voltage -- under no load. The problem is that under no real load, the capacity was there; however, when you loaded-up the circuits it didn't have the capacity to carry the amps because the connection wasn't good. It turned-out to be a problem with the ground continuity -- but it could have been the positive too, I suppose.

Under load, you might check the cables to see if they are heating up. You might also try to see if you're gett the amperage capacity under load.