Actually this problem started while trucks were still under warranty, Had local dealer send their tech out and that is how I found out what caused our problem. Had same problem on two trucks after warranty expired.
We had at the time nine 7fg's and at least five of them had this problem. Cooling system checked out ok and after we switched fuel all is fine. Have not had to repair any in over a year now since fuel change.
I was told directly from their tech that low grade fuel caused our problem.