Secure Channel problems

We have a long standing problem with a Windows 2003 Server and ISA Server. In
the System event log we are getting the following error:

"This computer was not able to set up a secure session with a domain
controller in domain NBC due to the following:
There are currently no logon servers available to service the logon request.
This may lead to authentication problems. Make sure that this computer is
connected to the network. If the problem persists, please contact your domain
administrator. "

The only way I can resolve this is to use netdom to reset the secure channel
with the domain.

This server has been rebuilt and not only that we were running ISA 2004 on
this server, but were having to reset the secure channel several times a day.
So we installed ISA 2000 and it seemed ok for months, now this has started
happening again. We have a Win2000 Domain. Does anyone have any ideas?