Something what also seems to happen with older cable modem's like Lancity or tetryon modems is that the loose sync with with the carier voltage over your cable
you could try a small service reset wich is quite simple with these older modems just remove the lan cable and disconect the power leave it for 5 min's and then reconnect it should resync itself with the carrier voltage and lisen to the dhcp info from your isp maybe that will be solve the prob if not my guess is either cabling or a defective modem