What points are you using, I have a distributor machine I can run them on and have found the new over the counter points are not worth much. I buy only NOS sets of Delco off e-bay, they work much better. The last Pontiac distributor I worked on would start missing around 4000 rpm, with NOS Delco points it would run up to 6300 ( limit of my machine ) and never miss a beat. I also use Jon's solid state condensers. You can actually hear the difference in points while running on m machine, the older sets have much heavier springs.
Check your distributor bushings, if they are loose, the points gap will always be changing. Replace your condenser with the solid state one, even the old NOS ones are questionable now.
