I found the rivet that holds the vacuum advance to the stamped metal piece clamped to the distributor had elongated its hole. This allowed the distributor to advance and retard uncontrolled a couple of degrees. Ended up drilling the rivet out, re-drilling the hole to make it round and using a 1/4" shoulder bolt.
I had two spare distributors and both were worn the same way, I’m wondering how many others have found the same problem
