I know you said you had a new electronic ign. box, if it is a motorcraft and it prob. is it still could be bad.......this is a classic sign of a weak box when it is hot....if you could pour so cold water on it when this happens you might elim. this.....
1979 CJ7 (YJ tub) full cage, Currie 9inch rear with Warn full float kit, detroit locker, 456 gears both ends, Lockrite in front, T 18 trans, 360, 040 over, Isky cam, 2 into 1 Flowmaster exhaust
I'm not as good as I once was....but I'm good once as I ever was.