You are correct in that I am watching the IP traffic from my PC and not at the OS.
I too did not see anything odd in the IP traffic until it stopped.
Mostly I am attempting to insert suitable debug messages in the firmware that will
tell me what is wrong. Doing this type of printf debugging is tricky because
you effect the timing. The printf’s can be a heavy load as they are synchronous.
So far it seems that the OS stops receiving messages when the hang occurs.
I don’t yet know why.
I am using the latest enc28j60 drivers which have some significant changes
by the developers. I had hoped that this would solve the problem, but it
is no better than the old version with regard to hanging.