Once you find the true Knocking Gremlin you may wish to add one of these:
http://shop.pertronix.com/pertronix-...ription_tabbed
Most of your stock vac advances all came with different operating specs, the Pertronix unit you can spec it to your application.
The best thing about those is that they are Ethanol Resistant Diaphragms. I used to go through a vac advance every 6 months!