Are you sure that you are hooking the timing light to the right spark plug wire? It needs #1. -The farthest forward cylinder.
Does your timing light have the dial advancement?
Some of these type that are not well made have caused problems for other people.
Try another timing light, if you are attaching to #1.

BTW the rubber can allow the outer ring of the balancer to "walk" around the rubber, throwing the marks helter-skelter.

Is the mark erratic like the timing chain is stretched? I have seen the chain jump a tooth, then someone moved the distributor to "compensate" for the out of time cam.