Constant errors: The master browser is stopping or an election is being forced.

We have 3 x Windows 2003 domain controllers.  1 is at my site (which I look after with great care), and the other 2 are across the road where somebody else is responsible for them.  There are numerous problems, but at the moment, every hour an entry appears in the event log of my DC.  It says:

   Source: MRxSmb, Event ID: 8003
   The master browser has received a server announcement from the computer DC2  that
   believes that it is the master browser for the domain on transport NetBT_Tcpip_{149BB490-CBE2-4A29-.
   The master browser is stopping or an election is being forced.

I've looked on EventID.Net but not a lot seems applicable to my situation.  There is one thing, however.  My DC has a subnet mask of 255.255.255.0, and the two different subnets I look after can access it because there is a gateway on each subnet.  The other DC, however has a subnet mask of 255.255.0.0.  Over the road, they have only 4 subnets so I can't understand why they need this mask?  They *should* have gateways like I do.

Can the difference in subnet mask be responsible for this problem?  If not, any ideas how I can troubleshoot it?

Many thanks,
Steve