Poor hot idle caused by ethanol gas? Whaddya think?

Hi Guys/Gals: I could use your advice on a lagging problem with my car. I have a very stock 1974 Duster with the stock rebuilt Holley 1945 carb. Stock, correctly sized base spacer gasket. The idle speed and mixture are correctly adjusted, and the timing is set to 10 degrees before. New plugs/wires and nearly everything else under the hood. When I cold start the car, for example after sitting all night in 40 degrees in the garage, she fires right up and goes to high idle. Kicking it down one minute later, it settles into a very nice curb idle. Things are different in the summer. After a hot drive on a 90 degree day, the car chugs (misfires) a bit at idle. Sometimes, it dies at idle when it's really hot outside, and it has a rough idle when stopped at a red light. I cannot find anything mechanically wrong with the car, or with any of the underhood settings and adjustments. I know that the carb is sitting on top of the hot exhaust manifold, but I think it should still idle smoothly without misfiring or dying out at idle after a hot day drive. Is it possible that the fuel that I'm using (87 octane unleaded, 15% ethanol) isn't good enough to keep the engine idling smoothly? What do you think would happen if I used ethanol free 91 octane gas? Would this prevent misfiring at idle? The engine seems to run just fine at highway speeds. Please weigh in with your opinion. Slant Six Dan: Any thoughts on your part? Thanks everyone!