For me, it sounds like netwrok problem.
check your machine and router/switch set up
i.e.
your machine set up for 10M-half-duplex and router (or switch) side is set up for 100M-full-duplex. This may cause machine up and down when it is trying to talk to NIS/NFS servers.
command "ndd" can help you.
do not forget to check /etc/system, sometimes, unix admin. will set up interface link mode on this file.
Hope this help.