Anyone have an idea of WTF might be causing this issue?

[TheIrishman]

Here's the short and sweet version. The 4.3L is based off of the old 350ci block. Retrofitting it to a roller cam was less than optimal. The wear you are seeing is from the camshaft walking back and forth in its journals and putting pressure on the distributor drive gear. In turn it puts an un-even pressure on the distributor shaft which causes your rotor to run in the slightest bit eccentric. Keep changing the cap and rotor and get another 60K out of the bottom end.

[Precision]

couldn't you put in one of those optical cap and rotor kits. I have one in my 66 mustang. damn chinese caps and rotors were only getting me 15-2500 miles.

optical one has over 15,000 with no issues.