I am away from home this week but like to respond to one inquiry from Saumil - Actually, air metering place DOES oscillate in sync with RPM. I think computer is sending signals to Idle Air Control Valve to open and close. If I simply unplug Idle Air Control Valve, then the car idles perfectly at around 7-800rpm. Not even a fluctuation to worth mentioning. Of course, with Idle Air Control Valve disconnected, it will never goes into warm-up high idle either and cold start takes little more cranking.

Occurrence of his rapid idle oscillation is not consistent and independent of temp. At one time, it idles very well for some time and start doing up and down when motor had been warmed up for some time.

Because it idles perfectly when Idle Air Control Valve is disconnected, I think it is an electrical gremlin of some sort...

Re. PS2CHO's comments - I never suspected the crank position sensor. Let me look into it.