UPDATE:
I replaced the RPM sender (gsxr, yes, it took longer than expected to follow the factory wiring route), things returned back to normal for the first two stops, at the third the tach died and the idle got rough. Any more ideas?

Thanks, John

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1986 190D, 2.5L, 5-speed swap, 180,000 Miles (60K by me).
Jeep CJ-7 with Cummins 4BT/NV4500/AtlastII 4.3.
Grand Wagoneer 4BT project in progress!