AirPort Express not working properly

Hi everyone,

I have a AirPort Extreme (running on 7.3.9) downstairs and my connection's speed downstairs is around 120 Mbps. I use an Airport Express (running on 7.6.9) to extend my wifi upstairs and but it's not that far, there's about 20ft and 2 walls between the Extreme and the Express. The connection upstairs is really unstable going from 1Mbps and peaking at 25 Mbps.The Airport Express often disconnect but continues to show the green light, so I always disconnect it, then reconnect the cable. I read on forums that the Express can only extend the connection's speed that he's getting so I tried putting the Express closer to the Extreme, where I would normally get 90Mbps without the Express, and it fails to connect to the Extreme.

Is there something wrong with my Airport Express or is an unstable and weak connection what I should expect from it?

Thanks