Try reinstalling the drivers from scratch. you would need to go into device drivers and deleting the device. Then reboot and let it try to reinstall the device.
Assuming your network card is capable of CAT6 speeds, you might go to the hardware vendor’s website. This situation has been known to happen when network card drivers are not the latest and greatest or when they have a bug in them for a situation.
If the network card is not built into the board and is in a PCI, ISA, or PCI-X slot, try another slot on the mother board. If this still does not work you need to remove the card and try another if it is removable, or disable it in BIOS if it is internal and then try installing a secondary card.