Win98 DHCP clients, wrong subnet mask

We have 2 Win98(SE) PCs on the network using DHCP. A few times each week one or other of the Win98 PCs reports an IP conflict. This is always because the PC has the wrong subnet mask. Releasing and renewing the address corrects this.
All other clients (approx 300 NT4, Win95 and apple mac) are fine. The only problems are with the 2 Win98 clients.

Any ideas?