Microwave ovens are a huge problem as well as wireless CCTV cameras. Maybe you or a neighbour are using one of those?
My wireless (N draft) usually runs at 270mbps on the pc downstairs, although if i switch the microwave on, it drops to 81mbps!
Changing channels has helped me, as the router used to find the channels automatically, always selecting 1, so I changed it to 6, the effect is still there but not as bad, drops to 120mbps instead.