Outlook will not connect to Exchange server after coming back from remote office on same network.

Running Exchange 2007; clients are on Outlook 2003. User has no problems connecting to Exchange server while at the office. User goes to another site at the same company and logging onto the same domain. When user returns to home office, he is able to connect to Exchange for and hour or two and then Outlook disconnects and is unable to connect to Exchange server. If left alone, it eventually starts working after anywhere from 2-6 hours. I can delete the mail profile, add a new profile with my username and it tries to connect but the user does not have access to my mail so no mail comes up. I can then delete that mail profile, add User's mail profile back and it then connects.....but most of the time, it will drop the connection again after an hour or so. What could possibly be causing this? Exchange mailbox store is at home office only, and there is a DC at each remote location. AD replication is, for the most part, 15 min intervals. I should also mention that this was happening before we upgraded to Exchange 2007 and were still on Exchange 2003 with Outlook 2003 clients. Please ask what other information you may need to help me resolve this issue.