I would be willing to bet that it has another problem, electrical, that
"playing with the ECU" solved by accident. There's an ignition wire either
partly disconnected or grounding out somewhere, maybe the wire from the Hall
sender, and messing around with it accidentally makes it contact correctly
again. Even the pin connector to the ECU might be slightly funky. Try
starting the van and playing around with the various wires, ECU, etc., and
see if you can  get it to stall or drop revs as you described. This problem
is not related to a bad O2 sensor. It could be due to the ground strap.

I'd wiggle the pin connector to the ECU around and if you can get it to
stall, that's your problem. What else but a bad contact could "the ECU lying
on it's side" fix?