There has been intermittent problems where the brass nozzle would be pushed out of the PEEK nozzle holder without the hot-end being overheated. As the problem was intermittent it was very difficult to isolate. I am now confident that the problem has been found and corrected.
In summary, it was discovered that some PEEK nozzle holders had a very very slight taper in the bore. As a result, the threads at the nozzle end did not quite grip properly and would sometimes release the nozzle. (The brass nozzle would, however, feel like it properly tightened down.) I discovered that this is due to the wear on a tool holding socket on the turret lathe. One of the sockets is worn and resulted in the drill bit being slightly off-center. As a result, drill bit would drill a slightly tapered hole. This resulted in the threads not having the support that they otherwise should have had.
In order to correct the problem, only the best tool socket will be used for drilling the bore on the PEEK nozzle holders. The worn tooling socket will only contain tooling that can be adjusted for this sort of wear.
If anyone has a J-Head hot-end that has failed in such a fashion, and is 110% sure that it was not due to being overheated, please contact me at firstname.lastname@example.org. As long as I made the hot-end, I will gladly replace the PEEK nozzle holder free of charge.