Not saying this is your problem, just sharing an experience.

I had given my son my 88 W250, 318 TBI, it did the same type deal you are talking about, but it would cut off when he hit a bump, had a lift kit on it. Not all the time, just sometimes. Anyway, he found inside the distributor a condenser, not sure right terminology, but it was rusted thru and grounding to the housing. Replaced it and all was well. Pretty sure that is where it was, been a few years ago.

Truck is still going strong. Best truck I ever owned.