Fishy68,the balancer is covered in paint by P.O.I,ll check to see if paint is cracked,moved?If not,how much does this shaft bushing move to cause irratic timing shift?Does this happen often?I guess I,ll be pulling the intake?
Yeah check the balancer like Johnny said. They do slip from time to time. The bushing being worn out is common on old blocks and mine was worn when I bought my car and it let the timing jump around about 4 degrees but I imagine the worse it is the more it will vary. The bad news is you have to drive it out from the bottom side upwards and have a special tool to burnish it in. I think I'd try one of the Hughes collars in the link Mad Dart showed. That'd be an easy fix if it works.
