If your losing the lease you might need to configure your modem. Either a longer lease or setting it to 'eternity'when possible. Try to experiment with the leases. If the clients are losing the Ip and the DIR is set to be the DHCP erver, you can try different lease settings on the http://192.168.x.x/Basic/Network.shtml page.
Also, there's a "always broadcast' setting on that page you might ant to try (doesn't fix a lease issue with a modem which isn't surprising since having 2 DHCP servers on the LAN is potentially disastrous. When the DIR is the DHCP server, set it to off on your modem (unless you know how to configure multiple subnets in your LAN))