on another van I have, '88 Carat, the *"stumble and running rich"* symptom
appears only when the van goes over some bumps or uneven roads and it
smoothed itself out when the road is flat again...

I found that the AFM harness which came out of the main harness is too
tight for some reason.. I pulled the main harness into the engine bay a
couple inches and the problem disappeared... which means too much tension
on that AFM harness

may be the AFM harness on your van is loose at the pins or one of the pin?
I would suggest that you pull it out of the AFM, take a small flat screw
driver and push the pins closer to each side... and check that there's no
tension on it.

good luck