DHCP incorrect subnet mask

I have a windows 2000 server running SP4. I have several
IBM Compatible machines in one lab that have troucle
renewing their IP address with DHCP. At the half life of
the IP lease when when they are rebooted they get the
correct IP with an incorrect Subnet mask. They are
suppose to be getting a 172.16.x.x IP and 255.255.0.0
subnet. They get a 172.16.x.x ip and a 255.0.0.0 subnet.
you can use Winipcfg and do a release and renew and they
get the correct one, it seems to be during power on that
they have the issue. The lease time is set to expire in
10 days so the issue only arrises once every week but it
is a huge annoyance. Any Sugestions?