I don't know. I'm not sure whether there is a setting to control the
timeout in Netbios. I seem to remember from years past that when a logon
token is generated, it stays working for many hours, even with Windows 2000.
For your clients and servers that only need to support connections from
Windows 2000 and newer, you may need to disable Netbios over TCP/IP in the
network card settings under TCP/IP, advanced. Since this setting is
presumably set per network adapter and not per computer, I'm not sure
whether it's very easy to automate this remotely via Group Policy or script.
Test it first to see whether it fixes the problem.
In ethereal, netbios would generate TCP 139 and maybe UDP 138. I think
kerberos would involve TCP/UDP ports 88 and/or 445. Things are slightly
complicated by the difficulty of running ethereal on a computer while you
log in, so you could either sniff on the server ,or sniff while you connect
to a server after logging in and being locked out, or plug two computers
into a hub and sniff from one while logging into Windows on the other.