I am having the exact same issue-- brand new computer with Gigabyte P35-DS3L w/ Realtek nic onboard. I'm starting to think the nic itself is the culprit. After about 12 hours troubleshooting Vista I installed XP and see the same problem. Note: I updated the bios on the MB, and tried the newest Realtek drivers from both the Gigabyte and Realtek sites (the Realtek site has a newer version than the one available from Gigabyte).

Please let me know (or post on this board) if your problem is resolved.