If you have access to a CAT5 cable checker, use that before you pull a new cable. To me, it sounds like one of the cable pairs has a bad connection, or break. I you have another machine you can plug into that jack, see if it does the same thing. If it does, most definately the cable. Also, don't forget to check the other side of the punchdowns, where it goes into the switch. Could be a prob there as well.