Subnet Mask

I have a windows 2000 server on my class C network running DHCP. My clients about 50 machine are running win98. What is happening is that the client machine’s subnet mask keeps changing from 255.255.255.0 to either 255.0.0.0 or 255.255.0.0. When this happen they cannot see other machines on the network and they receive a message that “no domain controller was available to validate the passowrd”. what is causing this problem and how can I resolve it? I thought the problem was caused by my network class. I had a class A address with a class C subnet. But I have since changed to a straight class C and the problem is still occuring on some of the clients. Is this a problem with the clients or a settings on the DHCP Server?
Thank you foryour assistance.