Slow signal through Express

My internet signal periodically becomes exceptionally slow – practically dial-up speed. My set is, internet through DSL. That goes through a modem to which is connected an Airport Extreme, At the other end of my apartment, I have an Airport Express. I always get three-bar signals everywhere so that is notthe problem. Occasioanlly, my internet feed will slow down but that's not the problem. When I first gotthe Express, this never happened. Recenly, however, I have noticed that I ill get an excuciatingly slow internet connection in the end of the house served by the Express on all devices. However, if I move to the other end of the apartment where the Extreme is, the connection is fine. It seems clear the problem is with my Express. Any idea what could be causing this? I've had the Express about six months and this problem only began to occur a couple weeks ago, And it is not constant but intermittent.