Varying spark, out of ideas

If you have a lean-burn distributor or a junker that you can lock everything out of, try that and see if it stabilizes.

I second checking the damper ring, and also....where did you get your parts? (don't say O'rielly or Autozone or Advance, please). I've had new distributors from AZ that had locked out advance cans and missing advance weights and all sorts of screwery.