IE6 Fallback to NTLM

[Jobo]

IE (6) and Kerberos

At some (actually one) locations in our network (which is spread all 
over the Netherlands) we have the problem that IE6 randomly falls back 
to NTLM, while FF keeps on working flawlessly.

Does anybody has a clou what is happening? Tickets are valid and 
available, and when a new instance of IE is opened, everything works OK 
again.

The facts:
Server: SLES 10 + Apache + mod_auth_kerb (Kerberos 5 release 1.4.3)
Client: IE6 on XP
Tickets are served by Active Directory.

Thanks in advance, Johan Bosma (j.bosma (at) mindef.nl)