Sounds like a bug in the device driver for the ethernet. Be sure you are running the ones you download from the manufactures website. Many times the ones that microsoft loads are generic and only "mostly" work. I do know there is a common bug called TCP offload that messes up a lot of things. You could try to turn that off..it is buried deep in the configuration menus for the ethernet controller if its there at all.
Would just installing a new ethernet card fix this since the controller is integrated into the mother board??