I have a funky one that I can't figure out. I have a server that is a standalone SBS 2000 server with a little lan (4 computers), and I get this funked out problem. After about 12 hours or so, the network connections are reduced to a crawling pace. A network drive that would normally take 1-2 seconds to pop open will take about 90 seconds. I have checked that system does NOT go into hibernate or standby. System network controller is also set at 100 / Full as opposed to auto. Network cards are 3c90b 3com 100 ethernet on server - linksys 8 port fast ethernet switch - broadcom 100 mbps integrated dell chip. Everything else works at regular speed however, AKA internet, accessing shares on other computers.
The only thing that seems to fix the problem is rebooting the workstation, at which point it is happy to talk with the server.
I hope someone has the solution, I've searched the site and gone through a couple of beers over the last few months trying to fix this one off and on!