I had this happen to me a few times, figured I was going to dig into it this winter and figure out what was going on. As a matter of fact, I had a few weird electrical issues happening.
Do you have a battery cutoff switch? I ended up removing the cutoff switch and bolting the positive wires directly together. All my issues have cleared up (now as I write this, I'll probably go out there and they will all show up again) but it's something easy to try and see if it helps.
Going to buy a new switch and install this winter. The contacts looked a bit oxidized on my old one.
Best of luck!