Recently I purchased a new computer and with it a new wireless router. After setting everything up, however, I began experiencing headache inducing lag at some of the most inopportune moments. It has been a problem with torrents, direct downloads, Counter-strike, and Team Fortress 2.  
 
The strange thing is though the lag isn't always consistent. For example: occasionally I will be playing TF2 and my ping will be between 15 and 50. Then for (what appears to be) no reason it will jump to 500 or greater and stay there between a couple minutes and a couple hours. Another example is UTorrent. I will be running a few torrents on it and one may be doing well, while the other will never reach above 20kb/s. (This is despite the particular torrent being capable of going much higher.) Even now, I am downloading HL2:EP2 from steam and the rate won't reach over 20kb/s.
 
Here is what my set up is (if you need more information just ask):
 
- Cable Connection 10MB down/3MB up
- SB5120 Surfboard Cable Modem
- Dir-655 D-Link Wireless router. (hooked to my computer through the lan port)