Win 98 loosing IP Connection

Hi, I have a school with IBM P2s and IBM P4s. They are
connected to a Win 2000 server. The server is running
DHCP. After a period of time, could be a few hours, the
workstations seem to loose IP connection. You can release
and renew the IP and they start hitting the network and
internet again. They have the same workgroup name as the
domain. The WINS address pops up in the winipcfg box and
is correct. They all have unique names.

Any ideas? Thanks!