OK, I got one of the fancy Belkin Pre-N routers in my mom's townhouse
so she could move her Windows XP HP laptop around and maintain
internet. The problem seems to be that the internet connection
fluctuates from 20+ Mbps down to disconnecting (no internet
connection). This seems to especially happen in the evening. There are
at least one or two other wifi routers broadcasting in nearby
townhouses. I'm not at the problem machine now but if my memory is
correct when the connection drops, sometimes even the connection
between the machine and the router is broken (i.e. can't login to
router from laptop), other times its just the internet and the
connection to the router is maintained (can login to the router but
internet connection says disconnected). At that point I can plugin the
laptop via ethernet cable to the router and get internet. Sometimes if
I reset the router all gets fixed...

I assume this is an interference issue. How can I troubleshoot this to
determine what is causing these drops or what appliance is causing
interference? Would a wifi network detecting device help? If the
neighbor's wifi routers are always on, shouldn't they always cause
interference, I mean wouldn't there be no daytime/nighttime pattern to
the issue?

I had called Belkin support on this and they suggested changing
channels to certain specific channels instead of using auto. This did
not mitigate the issue at all. I tried several different channels

Any advice would be appreciated.