I have just the same sort of problem with my Linksys wireless kit, compounded by having 3 ft thick Yorkshire stone walls.

I found eventually that if I set the four units up carefully so that only doors and the odd plasterboard wall were between the units, i.e. line of sight excluded going directly through stone, it all works fine.

As I said in another thread, the only problems left are that the microwave stops the rio - receiver in the kitchen and the wireless doorbell rings when one of the computers boots up!