Reving up and down at idle

[jackbof]

Hi all,

I have a 1975 350SE and it will idle nice and smooth when I start it but as soon as I touch the fan AC fan speed control ever so slightly it will start to rev up and down continuously. If I turn the fan off and fiddle around with the controls a bit it will return to a smooth idle. Sometimes while driving it will just do it randomly at idle but its smooth when throttle is open.
Any ideas on what might be causing this?
Thanks

[Randys01]

For a start have a look at the a/con vacuum advance  compensator which is on the firewall engine side . It is an electric solenoid connected to the distributor.