We use a Fritz repeater which should work exactly the same as yours.
Reconfigure the extender by disconnecting it and reconnecting it by pressing the WPS button or what ever method you are using. After that, change the channels your router is currently sending on, pick one that is not busy. I currently use channel 8, to see which channels are currently the most busiest around you use inSSIDer.
And lastly, upgrade your router OS. This all can be done on your routers configuration page (it's
http://fritz.box for me).
Good luck!