Cycles, yes, but what we have right now is more of a spiral - there's clearly some kind pf a positive feedback loop in the system that consistently drags parties apart, and that wasn't there before.
I think it started somewhere during the Reagan admin, going very slowly but steadily at first, but the wider the gap between the parties is, the faster the process goes, so it became really visible somewhere around Obama. The relevant rule changes wrt primaries and conventions date back to early 70s, though.
