internal and external same domain name web site display problems

Hello all,

we have small office with active directory, dns and exchange server 2003 on single server. internal and external domain names are the same "mydomain.com". I have setup static host a record in internal dns server: mydomain.com forward lookup zone with "www"  name so users could access company website hosted externally. this host a record points to external ip address of mycompany.com internet website. everything worked good for long until internet website reconstruction. website is developed under adobe flash. when users try to navigate www.mycompany.com it displays background color of the website with 0% (or 100% on some machines, or browsers) and do not load contents.
I tried to clear browsers cache, adobe flash player cache. cleared cache on local dns server, restarted dns service, but issue  not resolved. it i manually change dns server ip address from internal dns server to external, then everything works. something wrong with internal dns server, though the iP address of the external website haven't changed.

pls, help, how can i resolve this problem.