Stew, if it starts and runs fine when cold, then logically (as already
mentioned) you should start by checking the temp sensor and its wiring to
the ECU.  This is not the temp sensor your gauge uses, it's the one the ECU
reads to determine when your van is warm.  Once the van is warm this sensor
is the signal for the ECU to start reading your O2 sensor output and fine
tuning the fuel mix based on its voltage.  A bad sensor can keep the engine
in open loop operation, with a warm rich condition resulting.  The temp
sensor is cheap.

Fix your fuel tank.  I'm guessing you don't want a fire :-)  I had a
similar problem with leaking fuel when full, replaced the rusty tank, and
still had a leak after the new tank and seals were replaced by my shop.
One of the new vent lines was improperly installed and had pulled out
during reinstallation.  Make sure that you get a gas tank kit with all the
grommets, and fittings required.  I bought one from the friendly folks at
www.volkscafe.com.

While you're in there, you should also check the expansion tanks in the
rear fender wells.  Several listees have reported that the vent line
grommet/seal had been degraded, or dropped into the tank allowing crud into
the tanks...and subsequently the fuel tank.

Post your solution when you've figured it out.