<< I appreciate the suggestions for helping get this van to cool 
properly...>>

We have a cranky '83 that did the same thing for over a YEAR.  We 
learned to cope with the overheating at lights -- racing the engine a 
bit, making sure we don't drive anywhere where we have to stop, you 
know...

We finally put it in the shop, and it took them over a WEEK to go 
through the list of possibilities.  

It turned out to be the rear Bypass valve at the top of the engine.

It had broken and "rigged" by my husband over a year ago.

It's so small, if it clogs, or malfunctions, it won't help any to flush 
out the engine.  

It was a quick, cheap repair, and the bus hasn't overheated since.  It's 
a joy to drive.

Good luck.

Nancy