Dunno how you test that. Might be a bug in the firmware in the BIOS for the NIC. Upgrading/downgrading BIOS might change something. Look around on other forums for people with similar problems. Also what Rev of the motherboard are you on? It is printed on the board down on the left edge bye the last PCIe slot. I have my Ph2 CPU at stock 3.4 GHz. Pushing files consume very little CPU though, at least on Linux.
TweakTown has a forum with lots of BIOS tweaking/troubleshooting, might be worth a look/search.