If you clients periodically loose connectivity to your PDC via NetBIOS based commands, check the PDC event logs. If you find the following events, uninstall your anti-virus software:

Event ID: 3013                              Description: The redirector has timed out to <ServerName>                              Event ID: 2022                              Source: SRV                              Type: Error                              Description: The server was unable to find a free connection <number> times in the last <number> seconds.
NOTE: McAfee NetShield has been known to cause this problem.