In the continuing saga of trying to find the source for the low idle speed when restarting a warm engine, here are findings in response to George's suggestions and questions:
--Throttle plate is fitted squarely and snugly. It closes properly.
--No vacuum leaks either in hoses or at throttle
--Advance is set about 11 degrees BTDC with vacuum hose connected. This car has a vacuum retard distributor. Points and condensor have no more than 2-3K miles.
--The air slide is working properly.

This weekend, I checked all sensors from the ECU end. Everything fine.
I think the problem is spark related because I changed spark plugs and within five miles they were black. I think what's happening is this: Start up is just fine. After some running, and especially if the car is at idle, the plugs get overly sooty, spark starts to fail and engine speed drops suddenly. If idle is increased to about 2000RPM and held for 20-30 seconds, soot is burned off and the idle returns to normal. Then the whole cycle will repeat. Nevertheless, the air/fuel ratio gauge, which is reading from an oxygen sensor located in the exhaust pipe just below its connection to the exhaust manifold, says the mix is fine. What is a logical way to work through everything related to spark.

All suggestions cheerfully accepted.