I had the same problem until I switched to a Linksys cable modem. This seems to be their new model.
http://www.newegg.com/Product/Produc...82E16825120001
Pity, because the old model was meant to stack with the WRT54G series.
http://www.newegg.com/Product/Produc...82E16825120001
Pity, because the old model was meant to stack with the WRT54G series.