Repeated outlook username prompt after 10 minutes...

I am currently setting up a SBS 2008 server.  I can connect to exchange from an enrolled computer, and from owa without problem.  I have installed a certificate with the names
remote.docgreg.com, autodiscover.docgreg.com, DOCGREG-SERVER.docgreg.local, and docgreg.com.
Initially, when connecting to the server using RPC (outside the network) I can connect to outlook.  After about 10 minutes, however, I can no longer do so.  I get the prompt for username and password to log onto DOCGREG-SERVER.docgreg.local and it is repeated indefinitely.  Rebooting the server resolves the problem, for about 10 minutes.
There seems to be something that is being overwritten somewhere after a restart.  I would appreciate any advice as to where this might be.
Of note, when I do a remote desktop, I get a certificate error (shows the self signed certificate), although when checking the server under terminal server, it shows the proper godaddy certificate I'm using.  Maybe this cert is being changed after 10 minutes?
I'm also questioning if the authentication method is being overwritten after some time.  What should the authentication methods be?  And under what web service?
Thanks
Greg