Make sure you have enabled spanning-tree portfast on host ports. Check for any NIC card incompatibilities(make sure NIC drivers are latest)
http://www.cisco.com/warp/public/473/46.html
As a troubleshooting step, try hardcoding speed/duplex and see if it makes any difference. If this does not help, let us know
*How many VLANs you have
*What software version you are running on the switch