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.