1975 Stingray: 350 / 330 gen 1 engine with Eldebrock 1408 carburator - infrequent engine erratic idling with automatic  choke.  Normal vacuum range,  engine timing 12° advance,  set idle to 780 rpms. No detectable leaks, 700r4 automatic transmission 

Burns 91 octane fuel, runs OK for awhile, then idles up to 1,100 rpms,  later idles down to 780.

Car driven 50 miles a week avg, stored in controlled environment. 

Open for comments!

Sounds like a possible vacuum leak. Leaks cold, then expands gasket so no leaks. Spray some starting fluid around intake and carb base. If RPMs increase, there's your problem. Possibly! 

I have not detected a vacuum leak, but its possible. Your remedy sounds logical, and in the 70s I used the method, just forgot!  I'll have to pickup a can of starting fluid. 

Thanks for your comment!

I have a similar issue, sort of…..when I cold start, the revs are instantly at 2300 and don’t come down.  This just suddenly started happening after many years of having no problems.  Anybody have an idea what’s going on here?

You might check your automatic choke assembly, linkage assy, coil may be sticking causing a malfunction.