You have a bad connection/lead from the alt to the battery.
I have had the same. All your tests dont show much as they are not done while a good load is on the system. I replaced the wire from the alt to the battery & it fixed the problem.
There is regularly threads on this same problem.

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving