Look for loose bushinghs at the top of the distributor. Are you running points, or an electronic ignition? A wobbly shaft, or a worn distributor cam will change the point dewll angle between cylinders. One degree of dwell change equals 2 degrees of timing change. If you can shake the dizzy shaft and see any wobble at all, that's way too much! Even a HEI can have problems with looseness, just not as dramatic as points.
Jerry
