It sounds like you've already tested the Z68 with shorter cables and the problem only shows up when you exceed about 60m. If that's the case then the problem would appear to be either: 1) your particular MB is defective, or 2) the Broadcom chipset can't support longer cable lengths.
Try another cable,
Try a quality PCI-e lan card with, say, an Intel or NEC chipset (not sure which is better),
Get a replacement Z68-pro3 MB
Choose a completely different MB