There are a few things that I can think of:
1. Permissions – Does everyone trying to use GP editor have the proper permissions to do so.
2. See if the Sysvol on the PDC is shared properly.
3. Does your domain controllers have multiple NICs? I do believe Microsoft has an issue with multi-homed Domain Controllers.
4. Make sure all of your DNS settings are correct. I’ve noticed some users that have a SAN connectec via iSCSI to the DC and the IP address is resolving to the SAN and not he proper IP address of the DC.