Check for congestion:
1)Dowload InSSIDer:
http://www.techspot.com/downloads/5936-inssider.html
Install it and select the proper adapter. Let it scan for 1-2 minutes
2) Note the channels used by your neighbors. Note which channels are most and least used. However the only non-overlapping channels are 1,6,and 11. (you can use others, however you're subject to, and cause interference to the main ones)
For Really congested environments with no clean-clear channels (non-used non-co and non-overlapping)
I recommend choosing the channel that has the lowest signal strength that's around... like if the least use 11 and the only channel 11 in use has a really low RSSI (like -80+dBm) Use ch 11.
3) Log into your routers control panel and go to (for example) Wireless Settings>2.4Ghz or Wireless Settings>5Ghz and should be in the main of both sections. Configure those to use the least congested or best case channel.
PROTIP: DISABLE 40MHZ CHANNEL WIDTH! It uses a 2nd channel for higher bandwidth, however it's an overlapping channel and it would subject you to interference and would cause interference to other networks!!!
4)Apply the new channel setting, use the scanning software again and test downloads.