Just to let everyone know, you help me solve the problem. After
investigating engine temperature related items, I discovered an engine
block mounted coolant temperature sensor, just under the distributor.
After replacing this sensor, which sends data to the ECU, my rpm problem
stopped. I was always checking the sensor you can see easily that sits in
from of the thermostat. I'm told that that one controls the cooling fan
(and probably the temp gage on the dash).

Thanks again for your help.