Airport Express: Works for a while then drops

I have an Airport Express extending my Time Capsule base station. I've moved it out to the garage, and it works pretty decently even though it's pretty far away from the base station.

But in order to get it working, I have to unplug/replug to get it to connect. It lasts fine for the few hours I'm around it. But the next day when I return, it has dropped completely and the yellow light starts flashing. Unplugging/replugging resets it and it connects back up fine.

Any idea what's going on?

Thanks!