If you are in brooklyn, you probably have a bunch of neighboring networks.
get
istumbler it will let you see all your neighbors wireless networks and their channels. you want to pick the biggest hole to put your network in. keep in mind that channels overlap, to be clear, you need to be 5 numbers away. so 1&6 or 3&9.
if your neighborhood is anything like mine (midtown manhattan), you will never be completely clear. you just want to aim for the biggest space with the weakest signals.
you internet speed shouldn't affect your network speed.
is it possible someone else is on your network?
do you have the guest network enabled? or roommates who might be torrenting or doing other big downloads.
check your network to see who is attached.
To monitor wireless clients:
Open AirPort Utility, located in the Utilities folder in the Applications folder on a computer using Mac OS X, and in Start > All Programs > AirPort on a Windows computer.
Select the wireless device youre setting up, and then click Manual Setup. Enter the password if necessary.
Click Advanced in the toolbar, and then click Logging & Statistics.
Click "Logs and Statistics," and then click Wireless Clients. You can monitor the signal, noise, and data rate of wireless clients on your network.
if you are torrenting, make sure you have your upload throttled. You want to take about 10% off your available speed, as maxing out your upload will kill your download.
also make sure all your coax connections are tight, all the way from the modem to the wall, including any splitters.
a few months ago, my internet was going in and out, i remembered seeing a time warner guy climbing on the handrails in our stairway a few days before, took me several chats with Time warner before i remembered that, and the next time i walked by and checked, he had left one of the connections loose.
but i've had the same problem with the cable connection going into the back of my modem.
you may not be able to get to connections in communal areas, but if someone new just moved in, that could be a problem. They may have also jumped on your guest network.