Go to this link and download inssider. It will show you all the other networks in range and what channel they are on.
Since people never change the default by default you may have another signal right on top of yours.
http://www.metageek.net/products/inssider/
^^ Wisdom here.
I lean towards a channel overlap in this situation. Spend some time diagnosing using the current tools at your disposal before throwing good money at a problem that may well persist or get worse by adding yet more wireless AP's into a limited spectrum of channels.
If there are 10 AP's within talking distance of each other, you very well may be out of channels. Adding more hardware just introduces one more channel overlap.
Make a list of every available AP you can see from various places in your home, and their relative signal strengths. Front of the house, back of the house, 2nd story, etc. Find out on what channel each one operates.
Then, assign an un-used channel to your AP. If there are no un-used ones, then choose the channel of the weakest AP that doesn't belong to you. Overlap with that one, and hopefully your computer will choose to associate with that AP when on that channel.
ETA: I had a similar problem when I moved into my current house... I couldn't use wireless effectively from 1 half of my house. I had no signal overlaps though. The solution was to run Cat5 through the attic to the far end of the house, and install a 2nd access point over there. Now, I can wander my entire property (1/3 acre cul-de-sac lot) from the 400sq ft disconnected workshop in the far SE corner all the way to the mailbox on the far NW corner, with excellent signal. I can even walk to the far end of the cul-de-sac and down the block a ways (100-125 yards), before the various cinder block walls of the private yards obfuscate my signal. I probably have the strongest Wi-Fi network in the neighborhood. I actually want to install a higher gain antenna on the roof of my workshop and see how far I can push my signal... possibly a parabolic pointed at the park nearby (1/2 mile or so).