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. =20

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 sensors 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.

Bob