I personally did not have a 6.4, like you I had a 7.3 Power Stroke with no problems. I have a friend who had a 6.4 and had a problem like you are having.
He started off with a rocker, and then it escalated to a lifter, then to something else, and then got rid of it. International Harvester made the 7.3 and Ford made the 6.4 I understand. Good luck with your repair.