Slow internet through linksys and asus routers

I used to have great connection with my Linksys Router wrt54gc. After more than a year of use, it started acting up. Connection became very slow until it can no longer connect to the internet. So I thought my router was already defective. I bought a new one, Asus WL-520GU. The shop where I bought it from tested both asus and linksys routers and both were working perfectly. So I set the asus router at home but the connection was still very slow, in both desktop (wired) and laptop (wireless). I tried connecting the PC directly to the modem and the internet connection is great. So now, I’m stuck with 2 routers, both are not working properly at my dsl connection.