Check the linkage that connects from the vaccum advance to the part inside the distributor that rotates when advance is applied. It's the connection inside the distributor and there's a post that goes thru a hole to connect the two. There's a keeper on the post that can come off which allows the two to come apart, allowing the part that rotates inside the distributor to float and it will drift back slowing the timing way down causing the issues you encountered. I've had it happen to me twice, both times on a recently rebuilt engine. I'd bet money on that being what's wrong.
Hope that helps,
Ron
Hope that helps,
Ron