JBrian,

I had a similar, if not the same, problem as you describe it happening in
my '91 gl auto-boxer. The van would start and run fine for awhile, then
after warming up (5 or 10 minutes) it would behave as you describe, dumping
extra fuel out of the exhaust with no power, missing etc. I guessed it
might have been a temp. sensor. I replaced the sensor in the thermostadt
housing (not the blue one for the temp gauge in the dash ) a black 2 pin
sensor. This solved the problem in my case. Hope this helps.

Rick Koller
'90 sycro-west
'91 gl