318 Poly Vibration

Start by doing this: Devise a way to short the spark on individual plugs. Some guys pull off plug wires, DO NOT do that, as an "open" plug wire can cause cross-fire to adjacent "holes."

Sometimes you can ground a clip / alligator lead to a small screwdriver. "Loosen" all the wires at the distro so they are just sitting in the towers. Use insulated fuse pullers to pull out a wire with one hand and slide your grounding probe in with the other.

Use a tach if at all possible and listen. One cylinder that does not "drop" the same as the others indicates a "dead" hole for whatever reason.

Compression test? Cylinder leak down?

Somebody mentioned distributor. I've seen distributors worn in such a way that the points would sort of "accidently" open on some cylinders and not others, as the shaft was that badly worn/ bent. The oppposite can be true, sometimes they won't CLOSE on some holes