Based solely on my own limited diesel experience...
I would ask how it's been since the last filter change in your old Racor? (And, did this happen before than time?)
I once had a similar problem that soon progressed to non-starting and it turned out to be a microscopic air leak somewhere in the two O rings or in one of the several plugs in the top of our 1988 OEM Racor filter assembly. Since that model was long ago replaced in the market by a newer model, I replaced it with the current one. Problem solved.
Admittedly this is only a long-range wild guess, but it takes only a very very small air leak to slow a diesel under load.
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