I had the same problem with my Xbox 360 ( on my old G wireless). I had to change to a wired connection, as the signal was too weak to keep up with the amount of data a game creates.
I have since upgraded to N, have just tested it and worked perfect for 15 mintues.
I think I will stick to my wired connection, though.
Just so you know - microwaves and cordless phones interfere with wireless connection.