So...regardless of the cooling issues, does it sound like the wandering idle might be caused by an intake or some other vacuum leak?
Has anyone had the same problem and fixed it?
Take off the air cleaner and put a rag or shop towel over the carb., see what happens to the idle speed. If it goes up, you have a leak somewhere. Reducing the amount of air coming in should offset the leak by making the A/F richer.
Have you tried to find a leak? Have you checked for exhaust leaks as well? Hear anything that might sound like an exhaust leak?
Have you removed any spark plugs? Maybe take 'em out and compare 'em. Obviously a light one is going to be the lean cylinder.
Though you did not mention any braking issues, I saw a pic in your cooling system thread that you have a brake booster. Those vacuum lines are notorious for leaking, so is the little valve on the booster itself. Maybe look those over for cracks.