CHECK ENGINE light is NOT on. If I had to guess, I would suspect a bad plug wire is the culprit. I'm not sure, however, since the engine is missing out badly at lower RPM's only. The timing problem seems worst when the van's transmission shifts into OD and the RPM's drop or while going up steeper inclines.

What's puzzling is the van runs fairly smoothly at higher RPM's while on level ground. Is there some sort of a massive air flow sensor on these engines? An oxygen sensor I should check? Any troubleshooting ideas would be much appreciated.

 

A missfire will NOT trip the light on an OBD1 vehicle
Sounds like a good tune-up is all you need to me

 

JCswm, I replaced the spark plug wires and it runs like new. Amazing.