Hey Jerry,
I put a brand new battery in a couple months ago, even though the older battery was still testing in the acceptable range.
Back then, I did the cranking tests that you mentioned, and the old battery did pass the test, staying above 9.5 volts for 20 seconds.
With a hot engine, it truly acts as if the battery is weak, but that is not the case. I recall a couple months ago, after I had installed the new starter/solenoid, and the older battery was testing good, I drove the truck to the county landfill, and turned off the engine. Unloaded trash, then the starter barely turned over when I tried to start.....someone gave me a jump with a running big SUV, and it still barely cranked over, but finally started.
The V. Reg. had been overcharging at around 15.5 volts, so I tried adjusting it down....but it was still putting out around 15 volts. In my mind, I just cannot think of anything else besides the 40 yr old V. Reg.
