As you suggest, try another device with a wireless card, borrow a laptop. I very much doubt the walls would cause any problem with the signal at such a small range. Other wireless devices may cause problems, on your router see if you can change the channel which the router is on.
Failing that you could take your laptop somewhere you know the wireless is ok and see if the problem is repeated there, if it works fine their its either your router or something else causing interference. Try swapping the router to see if its the router to try and nail it down to router or other problem.