2 things to check,

1. make sure your distributor bushing is good. the plastic gets hard and then cracks and breaks. i usually replace them with an aluminum pc. this stops the end play of the distributor and needs to be snug to only allow a few .002-.003 of movement.

2. rust where your seeing isn't critical. sure it would look nice shiny, but is the pertronix moving freely with the vacuum can? is the gap of the reluctor changing as the engine surges? make sure the gap is about .006-.008

3 make sure your weights can move freely and are not getting bound up. i always clean these and grease the pins so i now they move. on the pins you want to use a lithium grease not something heavy like wheel bearing grease.