NIC Card changes its speed automatically & stop responding?

I have a small LAN of about 20 systems with Windows 2003 on the server machine, My “Realtek RTL8168/8111 PCI-E Gigabit Ethernet NIC” on the server machine sometimes changes its speed from 100Mbps to 1.0GB and stop responding to network communications. I tried to fixed the speed to 100Mbps but when i get to NIC properties in the Device Manager, i can’t find the Advanced tab. Can anyone please guide me how can i fix this error, NIC is onboard and Board is Intel DG31PR. Windows 2003 with Service Pack 2 is installed and Domain, DNS & DHCP is running on the server….

Thanks in advance………..!!!!!!!!!!!!