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Don’t just check the battery ground connections.
I had a similar issue with my 73 when I first bought it.
The cable connections were tight and the battery was new and fully charged.
I noticed a hint of corrosion around the cable right around the insulation.
I peeled the insulation back a little to clean it up and found that even though the cable looked to be in excellent condition it was almost completely corroded through under the insulation.
I replaced the cable and it solved my problem.
That was caused by the.battery leaking down the cable. I'm not sure what year they started using the aluminum cable with copper cladding on them but that's when they really started to get bad so changing the cable is a really good idea anyway.