Restrict computers to use specific domain controllers

Hi,

We have 4 locations (single forest and single domain) and each of the location has 2 Domain controllers. We have a policy to not allow the workstations to access other location's resources even the Domain controllers hence we denied it in the firewall. We have noticed that sometimes some of the machines won't get the group policy updation properly (we are getting at least 5 calls on daily basis from the users - out of 2000 users) and the machines are random, restarting the machine will resolve the issue (we are in the guess of may be the client is trying to access the other location DC hence it's failing). We want to know is there any options available in the group policy to restrict the location users to use local domain controllers only for the GP policy update?

Note: We are using 4 AD sites and configured the IP subnets properly. Clients are Windows XP Professional with SP3 and the Domain controllers are Windows 2003 R2 with SP2. We have a strong very restricted GP settings configured in such as way that if the GP is not applied means they won't see any drives and hence they can't work in their machines at all.

Thanks in advance

Regards,
Vinu Kumar T K