DHCP : IP Conflict problem on Windows98

Recently I started get IP conflict error messages.
That happens only on Windows98 and never on XP stations. When I run winipcfg I see the normal Address but wrong mask sometimes it’s 255.255.255.255 sometimes 255.0.0.0. In reality we have 255.255.252.0. Release-Renew always solves the problem. There is no other DHCP on the site. There are only 1011 events appear in the event viewer.
The only change I made recently – installed new switches but there is no DHCP feature on them.
Any idea?