Undersized packets

I just replaced all of my old switches with new Cisco Catalyst 3550 Switches. Everything is working great except for my IBM AS/400. The port that its plugged into is getting collisons and undersized packets. I have switched port, swapped the patch cable and hard set the port to the setting of the mainframes network cards. Does anyone have any suggestions or things to try. I am getting some strange things where the line does not go done between the switch and the as/400 but some of the clients sessions will stop communicating with the mainframe.