Just a reminder that ISSIDER, as well as NetAnalyzer, are fine diagnostic tools that everyone should have and use. ISSIDER does a fine disclosure of what is broadcasting, but a very poor job of graphing. NetAnalyzer is just the opposite. Much better graphing but not as much actual signal detail.
What is important to remember though, is both only disclose what is broadcasting visibally in your immediate area. It will not show what is not visibally broadcasting (except for your own, known router).
Routers that are invisible will not show and therefore it is still likely that you may still have conflicting signals interfering with your wireless connection. Try other channels even if your analysis software says there is nobody using your channel. Other routers may be "invisible."