DHCP Issue

This is the wierdest thing I’ve ever seen, I run a network of about 130 PC’s mostly on DHCP, on a bunch of machines there is a problem where every once in a while for no apparent reason the Subnet mask changes from 255.255.0.0 to 255.255.255.0 or 255.0.0.0. I’ve used Linux DHCP, Windows DHCP DHCP from a 3Com firewall and they all have the same problem. If you release then renew the address it comes back fine. The only thing that I’ve seen that the users have done before this happens is printing from excel but that is not always the case. I’ve checked numerous times for the existance of a second DHCP server but there is none. Anyway if you can help I would appreciate it.