Hello,
I'm a relative noob when it comes to computing. However, I do have quite a knowledge base in radio; both practical and theoretical.
Nothing can be more frustrating than a problem which comes and goes. Especially when the fix is several floors away. I tend to agree with ITSSJFA ; the signal is too weak. If your budget allows, a used Linksys N router with dd-wrt firmware and a couple of hi-gain antennas should do the trick. I suggest the dd-wrt firmware because it allows the router's signal strength to be raised or lowered as needed. Additionally, with dd-wrt the router transmit/receive channel is user adjustable. Then you can get away from the more congested frequencies(channels). How will you know which frequencies are crowded? Again, the dd-wrt firmware gives the router the ability to scan the airwaves and through its web GUI let the operator "see" the other routers, their signal strength, type of encryption and which channel they're operating. I realize this is a simplistic explanation, therefore, hg3300, pm me if you want a complete explanation and router recommendations.
Good Nite,
DrBogo