Distributor seems to jump out of timing?

Good point. If it were jumping time there it would be either 180° out or correct.
Pull that distributor out and check for wear. Maybe the advance plate is floating around loose.
Another good point...some intermediate shafts have the gear pinned to it. If that pin is sheared it will appear intact until you give it a rev and it will slip. Again, never seen it happen on a SB but its a very common problem on /6 with the nylon gear.