340camshaft with stock conveter?

Acting the same way at idle with the vacuum advance disconnected is a good sign that you are on ported vacuum. What are the RPM's in park/neutral? This will give some indication if you are even possibly up on the mechanical advance in neutral idling, or not.

It is indeed possible to have a TC problem too. But I would not conclude that until the timing and tuning have been wrung out and you know exactly what you have there.

Eliminating a booster leak is easy; just pinch it or disconnect and plug the line.