I know you siad you set it manually. Have you tried to set your Ethernet Manually with set link speed 10 and/or 100 with and without half/full duplex. I have a different card and had to use 100 Full Duplex, with static ip, subnet and gateway.
There is a small possibility that your ethernet cable will not support 1GB or 100MB even. This can be from interference, kinks/shorts, crappy quality, even length. If you had Snow Leo or another OS with same hardware then I would say....

There may also be an issue with the "network driver" (aka kext), this may help:
http://www.osx86.net/view/378-realtek_rtl8139.html . If you want to go a harder way (more reliable) use a EFI string and put BuiltInEthernet=No in your chameleon boot.plist.
I hope this helps you!