Hi,
The two things spring to mind, the first would be to delete the internal A record from your domain controllers and secondly make sure that the forwarding server entries on the domain controllers are working correctly. If you haven't done so already I'd suggest pointing the forwarders to your ISP's Public DNS Servers rather thanks google for example.
This should get you out of a hole short term. But without looking at your lookup zones I couldn't be sure what's going on. We have nicked named DNS "Black Magic" in my department!
One last though, could it be someone to do with your Firewall which may be running DNS?
Regards
Archer
