I wonder if the loopback shutdown is a red herring.  The "files nis"
switch around seems more like a clue.  Perhaps some outstanding RPC
after the network shutdowns (ethx's are down before the loop, no?)

I'd try playing around the shutting down these bits manually in various
permutations leaving the system running to see if it hangs.

I have SL6.x systems running NIS without problems but then I did not
reverse the "files nis" in nsswitch.conf.