try installing its driver from vendor's site. It happened to me a couple of times, when the LAN driver is detected by the OS without installing the proper one. It seems to work for sometime, but it frequently disconnects and after a while it automatically connect. Installing the driver solved the issue. Also it wasnt showing 1GBps upon connecting to a gigabit switch, and 1/10 times it shows 1Gbps. With proper driver it working like a charm in 1Gbps.
If that doesnt solve the issue, get a PCI LAN card for 200/-. Gigabit cards are expensive, and you wont need it unless you are connecting it to a Gigabit network switch. 10/100 NIC is enough for normal LAN and internet connectivity.
Also i thik you havnt touched the PCI base click during OCing, as changing that from default 200mhz might render most PCI cards dead.