I’ve got that issue where the van runs fine cold then gets sluggish when warm.  Mileage is down to 15 in the city too.  When I disconnect the O2 sensor it runs better warm.  I connected the volt meter and while disconnected it ranged from 0V at cold to about 1 volt after a few minutes it pretty much stayed there.  Then I connected it back up and left the volt meter on. It went up and down from .15 to .17 at about 2000 RPMs and stayed in that range.

Thoughts....

Thanks,
Bryan