Unusal Network Problem

Brand new PC with XP pro Aopen M/B with onboard LAN. Network works fine on test bench when taken to location it wont even produce a link light with a phsyical connection. The connection has been tested with a notebook and another PC which both connect fine, the cable has also been changed. When bought back to the workshop it works as it should with the network, with no problems. Remember when connected at location there is not even a link light. Plus network card on the PC is loaded with no system reported faults.
Beats Me?