[olddog]

A Cam with a lot of valve overlap will idle with less manifold vacuum. You may need to adjust or change out the vacuum canister. The Vacuum advance should pull in 20 degrees of advance at idle. It should be pulled in solid at about 1 "Hg less than the engine idles. If not the idle is going hunt all over the place, because rpm changes vacuum, vacuum changes advance, and advance changes rpm. It will chase its tail.

You set the timing with the vacuum disconnected and plugged. Then when you plug it in you should see the timing advance 20 deg and it should be very stable.