Yes, Domain Controllers should be Master Browsers - setting your domain controllers as "Preferred Master" (registration wall; try archive) should fix most of the Master Browser election problem.

reg add HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Browser\Parameters /v IsDomainMaster /d True /t REG_SZ

One of the common reasons for operating a WINS server is to provide a central reference for NetBIOS names across several subnets. Choosing one primary WINS server and pointing all clients to it lets NetBIOS name resolution work across subnets without requiring the use of Domain Controllers.

But if your system is supposed to rely on domain controllers, loss of replication for site C may explain your symptoms. So start with verifying successful replication.

This can safely be ignored. It's the result of the Browser service trying to figure out which computer on the network subnet is the most trustworthy. It's a throwback to Windows NT 4 days.

In a peer-to-peer Windows network, Windows computers try to maintain a list of computers on the same network subnet. On each computer maintains it's own list, but also the computers will participate in a "master browser election" on the network which will determine who is the most authoritative computer. An election process can be called for by any computer, and computers seemingly call for elections at the drop of a hat.

If all the computers on your network were running NetBIOS/NetBEUI or IPX/SPX and you lacked a WINS server this would be a much more valuable service, but DNS has essentially completely replaced this service (as well as WINS). If you ever go to Network or My Network Places, the devices you see here were populated by the Browser Service. In my experience, this list is also generally incomplete. Windows 2000 computers don't talk well with Windows XP and later systems, for example, because protocol changed to TCP/IP. Browser doesn't work through most software firewalls, either.

The service is present on Windows Vista and 7, but it is not necessary and is safe to disable. It's there completely for legacy support.

What is Computer Browser Service?

In registry key:

\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services \Browser\Parameters

with this you can make one computer as a master browser and prevent the others from becoming master browser with IsDomainMaster="FALSE" MaintainServerlist="FALSE"

and make the machine you choose to become Master Browser with

IsDomainMaster="TRUE" MaintainServerlist="TRUE"