I had a problem like that once with a '75 23'. Finally found it was an intermittent connection on the fuse panel where DC power from one source came across the board on a pop rivet from the other side of the board. The rivet was loose.
Can't say if this is your problem, or not, but it's a place to start checking. I found mine with a volt meter tracing out where I had power, and where I did not when it was out.
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