[操作疑難] Router 成日話 lease is lost (from ISP) 點解

Router model : ASUS WL-530g
Firmware : 1.8.2.5

我個router每隔半或一小時佢裡面個log就會有以下訊息:
dhcp client: deconfig: lease is lost
唔夠一秒就會:
dhcp client: bound IP : 42.2.xx.xx  from 42.2.117.254
但就係因為呢個小小既動作已經令到download緊既野停左 (例如rapidshare/sendspace) , 之後down唔返,好麻煩,
update過firmware都係冇用.