Losing NT domain

Over the last week at around 2.00 am, the NT PDC is unreachable. We can
still ping it but can't logon onto it. When you press CTRL+ALT+DEL - there
is only the User name: and Password - missing the Domain option. If anyone
logs in on the domain - no scripts are run and we can't access the scripts or
any shares on the server. Can't launch user manager or server manager - it
complains ther is no PDC available. As we can't get into Server manmager
can't promote one of hte BDC's to a PDC. THe only option is to reboot the
server.
It's Windows NT 4 with Service pack 6a.

We found an error message on the server lastnight - Access Violation in
Lsass.exe and the application log reported

Event ID: 4097 - the application. exe\lsass.dbg, generated an application
error. The error occured on 11/29/2007 @ 2.0.4609 The exception generated
was c0000005 at address 77f64860 (RtlAllocateHeap).

I found couple of articles on KDB but one is for Windowss 2000 (267578) and
the other recommends installing Windows NT 4 SP4 (154087)

Help!

Thanks
Bindu