I've had issues like this before...

http://windows7forums.com/windows-7-networking/4958-no-network-access-after-restart-3.html

http://www.netsetman.com/index.php?s=nsm

Seems to be a Realtek (for me atleast) issue.  Sometimes the Realtek NIC's will crash under heavy load (lots of threads open, server stuff) and require a reboot to work again.  Intel and Atheros chipsets seem to be immune but whatever.

Anyway try those, see if they work for you.