My 1988 had this problem (starts fine, stumbles when warm) when I first got
it. A monstrous pain, compounded by my having even less knowledge then now.

My problem turned out to be the wire that carries the signal voltage from
the O2 sensor to the ECU. It's the green one, and the connector is in the
left side of the engine bay. It's a co-axial cable, unlike any in the rest
of the van, with a grounded shield outside and signal carrier inside (like
an audio cable). If there's any continuity at all between the shield and the
core, the ECU freaks out. You can check it with a multimeter, with the ECU
end disconnected.

Good luck!

Matt Sutton
Brooklyn NY
88 GL