engine cutting out down track after in 3rd gear!

Thought about this a bit more. Could your Rev Limiter be coming in early? I know the MSD units do but I don't have any experience with Pertronix units. Try upping your rev limiter to about 7200 and take it out for a whirl shifting where you normally do. If it does not happen then you found your issue.

Scratch this...... I see it happens around 4K RPM on the Fwy. Fuel or Ignition is my final judgement! Lol