If your vacuum advance is hooked up above the throtle plate, that means it only advances while driving. That is not what you want. You want vacuum advance during idle and light acceleration. Hook up the vacuum advance to manifold vacuum, then set your idle again, and try it.

Other than that, it would seem to me that you need more air cause it is only at idle that it heats up,.... so I would install a clutch fan with more blades on it.