Ellen,

I had this issue with my bus as well ('90 2.1l). I found that my issue was
the Temp II sensor - and also possibly the wiring that goes to the Temp II
sensor. Once I replaced the sensor I've had very few problems.

That being said I've seen many posts on this over the years and it seems
that everything on Ken's list has been mentioned as the possible issue.

Good luck - and welcome to the list.

--
Thanks,

Jeff
'90 Carat (Grover)
'86 (We call this one Parts)
'78 Bus (Melissa) Patty's Bus