if the vacuum advance moves the dist when you touch the gas in neutral it's fine, not a bad diaphragm, a check of that is disconnecting and reconnecting the line should make a just noticeable difference in idle speed - if you have good vacuum there it's doing it's job, the vacuum holds it correctly and loss of vacuum on opening the throttle causes the movement ... the other part of the advance that might cause a problem would be the spring tension on it affecting the centrifugal advance, but might be worth checking the timing again, or even adjusting it slightly by ear
Bill
