Really weird ignition issue...

Bewy said

I cannot see a 'rotor phasing problem'. If the phasing is wrong, the spark goes to the WRONG cyl. The OP says the engine has a great idle & great throttle response. You would not expect that if 8 cyls were getting spark at the wrong time....

Looking at the phasing pics of both dists in post #30, I do not see much difference, given the method used to measure the difference. The rotor tip has a considerable amount of rotational movement, relative to the pick up generating the spark. It moves with the centri weights & with vac adv.

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The phasing is close enough to work until the vac advance is applied causing a misfire. In a correctly phased distributor the rotor tip is slightly away from the distributor terminal and as vacuum is applied it is pulled towards the terminal and at full advance can land slightly past the terminal. If the distributor is incorrectly phased at the past the correct terminal position to start with and vac advance is applied it will pull the rotor away from the correct terminal and toward the terminal next to it. I suspect this is what is happening in his case. Incorrect phasing can be caused by several problems. Incorrect reluctor placement, Incorrect pick up coil placement, Incorrect rotor placement, Incorrect cap placement, reversed polarity on the pick up coil, and etc.