My problem sounded exactly like what you are seeing and it went COMPLETELY away when I switched to UNSHEILDED cables. Inspect the RJ45 connector jack at the cable end you are plugging into the Cisco, if it has a metal jacket on/around it my problem...
I encountered similar problems with both a -28P and a -10P but i FOUND the problem in my installtion. Are any of you with this problem using SHEILDED CATx cable? If you are then that is the problem. I ended up removing the shield jacket that is wr...