Have you tried adding grounds yet...?
Might want to ground alt frame directly to neg batt. term to eliminate that potential problem....also check voltage drops between all ground junction areas with DC meter on low voltage scale....with full load on system....One forum fellow had a bad transmission to frame large ground cable that was causing this exact problem.....that was on a 123 series....at least rule out ground path problems since the alt checks good on the bench test....

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1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)