[Jack]

Brithunter, do you have any means of measuring runout on a fired case?
A runout gauge would be ideal, but sometimes you can see significant amounts of runout- if they exist- by slowly rolling a fired case across a mirror. Watch the case neck- if you see it wobble with the naked eye as you roll it across a mirror, you've got significant runout.
I'm certain you know what I'm talking about, but for those who might not... runout of a fired case measures the (lack of) concentricity between the chamber and the bore. If the chamber is significantly out of line with the bore, well...you're screwed. You'll never get it to shoot really well.
Brithunter, nothing you've mentioned in any post really leads me to think you have a problem with chamber/bore runout, except... it's one of the few conditions you can run across in a used rifle that you cannot remedy short of rechambering or rebarreling.
And you've already done all the other things that usually cure rifles with accuracy problems.