Car will not return to idle!

Ok I have a Mighty Demon 850 mech. secondaries. You have to keep an eye on her when she is warming up to keep her from dying. Once the car is warm it idles at 1200rpm....which is where I want it, it has a pretty aggressive cam.
When you take the car out to drive after warm-up is when the idle seems to have a mind of its own. At the first stop light (maybe 2 miles of driving) the car will have an idle of 1500rpm.....then when I am approaching another stop after about 15 minutes of driving I will got to stop, and the idle will fall to 700rpm. Even when i blip the throttle it will not return to my origional set idle.
Other than that the car runs perfect.
This is a 4 speed car, and as soon as i push in the clutch the car just picks its own idle...lol

Very frustrating....:mad:

Install a good throttle return spring.

A sticking spark advance can cause the idle speed to do that.

Does it have a choke and fast idle cam? Could be the fast idle cam is not working properly.....