Problem with wireless connection

Hi guys,

A few months back I got ESAT BT broadband and decided to get a wireless router. I am now using the ZyXEL Prestige 660W router.
My wireless router is located in my hallway and I have no problem with my wireless connection when I use my laptop
from my sitting room. However from any where else in the house I can only sometimes establish a network connection and on the rare occasion that I do establish a connection it drops as soon as I try to connect to the internet etc. When in my kitchen my laptop would only be 18 feet or so from the router but the connection continually drops.

I have read the article on tomsnetworking referred to in the FAQ's and as a result slightly changed the location of my router but this has not improved the situation. Im not sure about interference, I don’t have a cordless phone and the microwave is never in use while I’m using my laptop.
I have used Netstumber to have a look at surrounding (visible) networks and there are 4 or 5 but none are on the same channel - I am using channel 6. If it is some kind of interference would it not exist all over the house?

Also I previously updated the firmware on my router.

Can anyone give me a few pointers as to why I might be having this problem or further analysis I could carry out to try identify the source of the problem?

Cheers
BB