I think I found the problem….
Theres a bug in my ubuntu version (8.04) and I think it isn’t fixed yet because I updated & upgraded my system but the problem still was there.
This bug causes the kernel to use a wrong driver for my NIC (Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller (rev 02)).
I have solved the problem with the help of this script: http://acm.cse.lehigh.edu:8080/~jhw204/hardy-r8168
After finding it mentioned here: https://bugs.launchpad.net/ubuntu/+source/linux-source-2.6.22/+bug/141343