Idle eratic swinging surge to stall

[rocknrock]

I have a 92 RoadMastr Wgn, has always been a smooth runner. This winter one day when started, the idle began to sway up and down, surging to 1300 RPM then dropping back to 800 RPM, almost stalling then surging to 1300 again. I searched this forum and found 1 old post from 2012 without an answer.
It must be a component that can just stop working? , because it was a sudden change and just started happening.

I found the distrib cap was quite corroded, so decided to do a tune up. Installed new spark plugs, new cap & rotor, and new distributor. (Note - the old rotor was stuck and when I tried to pry it up with a scewdriver I cracked the "reluctor"... had never heard of a "reluctor" before... sounds fake like "flux capacitor"... so that's why I had to replace the whole distributor.)
The surging idle issue still remained after... so tried replacing the "IAC valve" (also had never heard of before). Still no change.

Next tried changing the EGR valve.. no change in eratic idle, tach still swings from 800 to 1300 every 2 seconds or so. Car stays running so put a vacuum guage on it... swung from 13 to 20 back and forth with the idle.

Also tried carb cleaner around the base of the throttle body for vacuum leaks - no smoothing out of the idle - no change.
Inspected the throttle body injectors while running, both supplying equal amounts of fuel.
Throttle body cylinders are nice and shiney clean inside including the butterfly plates.

Not sure what to try next?.. and what could just all of a sudden change or break or stop working to cause this condition? It has always been a smooth runner.
Will welcome any suggestions. Thanks in advance.

 

[Griswold]

My 96 did something similar. It was the IAC (idle air control valve).

 

[rocknrock]

Thanks Griswold for replying. It's nice to know others have had this problem too. I have replaced the IAC valve already. I read someone else who said the issue was their EGR valve. I've replaced that too.