Download a copy of InSSIDer, and it should help you diagnose things at least.
Change Your Wi-Fi Router Channel to Optimize Your Wireless Signal - How-To Geek should give you the basics. You're looking to change your AP's channel so that it gives least interference with other wifi around the area.
Also, things like CB sideband, DECT phones, Bluetooth, etc can all degrade a wifi signal. Be mindful of where you site your AP too.
HTH
Mike.