Do you get network disconnections on Windows 2008 R2?
Does Exchange and authentication quit some time after the networking fails?
Could it be a resource issue?: http://blogs.technet.com/b/instan/archive/2009/07/06/rodc-s-and-port-exhaustion.aspx
Check ports status with netstat -anob to catch the process in the act: http://blogs.technet.com/b/askds/archive/2008/10/29/port-exhaustion-and-you-or-why-the-netstat-tool-is-your-friend.aspx
This is a good link: http://support.microsoft.com/kb/2028827
This is a good explanation of the TDI API: http://www.codemachine.com/article_tdi.html
Check this out: http://support.microsoft.com/default.aspx?scid=kb;EN-US;961775
Pulled from here: http://community.spiceworks.com/topic/103398-sbs-2008-exchange-issue
Winsock, mmmmmm.
Here is a nice post on other patches you might want to consider for W2K8:
http://blog.itoncloud.com/?p=616
In case it goes away:
KB974090 KB975512 KB979223 KB981603 KB981872 KB982860 KB983458 KB983528 KB983620 KB2028827