This is probably some sort of interference from another device. I would suggest trying channels 1, 6 and 11 on the wireless router. Those are the 3 main channels, all the others between those are overlapping. Try each channel for at least a couple hours and see which one has the least interference. The router placement can have a profound effect on interference and reception. Get the router as high as you can, and be sure it's at least 10 meters from any other source of radio, like portable phones and such. My macbook drops the wireless connection if I answer my cordless phone right next to it. Channel doesn't matter, since the phone is spread spectrum it uses multiple frequencies and they just interfere with the signal...
Another thing to think about when it comes to interference is the walls, pipes and internal wiring can have an effect on the wireless signal, I have seen dead spots behind power boxes and large internal pipes. Put the router on the highest point you can and try rotating it about 5 degrees every hour or so, and see if you can find a spot that eliminates the problem with your wireless. Take into consideration that as you move around with the laptop, you are bound to hit a dead spot or 2 in the house.
Chronic disconnects are typically interference from other devices. Is your router the only one you see in the list of networks? If you see other routers, chances are good they might be overlapping your signal, changing the channel you are on may or may not help, but try all 3 main channels, 1, 6, and 11.
