Addendum...I have not been able to confirm this does not happen in a network print mode as well.
Looking at the service manual I am curious if the duplex motor is overheating and stopping. I might think this would be a source of trouble. Don't know if any of you have seen this or not?
Printer is in a classified location, and I cannot access it at all times; it may be next week before I get to actually test and diagnose in person.