Win 98SE dhcp client – wrong subnet mask

The same problem was asked by someone else last year (https://www.techrepublic.com/forumqa/thread_detail.jhtml?thread_id=48706&src=search) but none of the solutions are appropriate for us.
Here’s the problem: We have several hundred workstations, and occasionally some of the Win98SE’s end up with the wrong subnet mask. (i.e. instead of 255.255.252.0 they get 255.255.0.0 or 255.0.0.0). We have only noticed it on Win98SE’s. We have ruled out switches, NIC’s, or workstation hardware as there are no consistent variables. We are using W2K DHCP server, but noticed earlier when we had NT 4 DHCP server.