Vacuum advance

Yup, I've had exactly this situation before, and the others have probably nailed it: too much total advance at idle because your vacuum advance is receiving vacuum at idle. This may be through no fault of your own; you may have hooked up the vacuum advance hose to the correct port on the carburetor, which may have been deliberately designed to supply vacuum to the advance at idle; that was a frequent difference between the otherwise-identical "With manual trans" vs. "With auto trans" carburetors for any given application. However, you've also advanced your base timing 5.5° more than the spec of 2.5°BTDC, and your vacuum advance unit may not be to original specs, which could all add up to the gasping and skipping at idle when you hook up the advance.