STEP One, Disconnect and plug the vacuum advance.
Step Two, Check and verify what the timing is doing at or near idle.
IE:
Initial timing is just that and it should immediately return to the same setting when blipping the throttle.
If it floats IE: slowly comes back down the mechanical advance inside the distributor is too loose and will drive you nuts while trying to establish a repeatable idle RPM. WHY??? Because the mechanical advance curve is not snapping back to its starting position.

Hope this makes sense ?? Start with the basic steps 1 & 2 and report back