Network freezes

We have an ethernet network with two Cisco Catalyst switches, Windows 2000 and NT 4 computers on an NT4 domain. We are experiencing random freezes (every 3-5 minutes) on every network connected machine, resulting in timeouts and lost connections. Several network monitor programs are not showing any excessive utilization or broadcasts. We have reset all our equipment, to no avail. Any suggestions?
Thanks to all in advance!