Hot idle speed oscillating

My 1990 Mitsubishi Eclipse (DSM 1G 2L non-turbo) is starts and idles cold fine, and runs fine warmed up. When warm and I put it in neutral (AT) the idle oscillates between 1800 and 2200 with about 1/2 second cycle time.
MAS was the first guess -- so I put a rebuilt mass airflow sensor assy in and same problem. Checked all the connections and vacuum - all OK.
Also took the throttle assy apart and cleaned all - helped the running but still oscillating. Cleaned Idle air motor, checked voltages and resistance for all senosors and also voltages coming from the computer, and all OK.
About out of ideas -- anyone have some thoughts on how to start troubleshooting this?
Thanks in advance.

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