I used to have a similar odd problem in my 77. Every once in a while (like once every 3-4 months) the blinker would stop working except for one direction and even then it would only blink once. I figured out somehow that by leaving the turn signal on, then turning the emergency flasher switch on for a couple flashes and back off again, the problem would go away for another few months. I never did find out what caused it, but I think after rewiring my rear light harness (dumbass thief chopped it stealing my taillight) it has never come back. I recall reworking the bed ground when I did that so perhaps that was the trouble.
Could be the taillights and running lights are somehow grounding back through the flasher circuit instead of to the bed/frame. When they are off, things appear to work, when they are on things go funky.
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