LSASS.EXE Hogs Win2K CPU

I have 3 Domain controllers running Win2000 that intermittently get bogged down with 100% CPU utilization. Process that takes 60-80% is LSASS.EXE. I have run Trend’s sysclean, scanned the servers for viruses and they are clean. All Win updates, including SP$ are installed and antivirus aptterns auntomatically update and are checked daily. 7 other servers on the network are not affected. Any ideas out there?