I have been receiving this Event 8003 on my AD Master.

Event Type: Error
Event Source: MRxSmb
Event Category: None
Event ID: 8003
Date: 20/04/2007
Time: 01:02:30
User: N/A
Computer: someserver
Description:
The master browser has received a server announcement from the computer SOMELAPTOP that believes that it is the master browser for the domain on transport NetBT_Tcpip_{ID}. The master browser is stopping or an election is being forced.
For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.

I found this thread that describes how to fix the problem, basically I need to disable the computer browser service on the offending laptop to prevent it taking part in elections. That should cure the problem.