Chevrolet Repair: idle not stable, air control valve, oxygen sensor

Question
HI van
I have a 87 astro cl which runs fine,starts fine,drives well but once it warm the idle will drop and then go back to normal and its not allways the same, some times just for a split second and some times a bit longer you can only tell  when you are stopped eg at the lights/signal.all plugs and leads are new

Answer
Hi Ivan,
If that engine is fuel injected, there are a lot of inputs that the ecm is trying to sort out, so it can adjust the fuel flow.
My thoughts are a contaminated oxygen sensor, causing it to respond slowly, or a dirty idle air control valve port.
The second can easily be cleaned, but the o2 sensor should be checked with a scan tool to verify its operation.

Van