Wireless problem

I am trying to get internet in my garage which is about 50 feet from my house and my wireless router. I am using a belkin n1 wireless router and a belkin n1 wireless usb adapter. So far I have not been able to get any connection using the usb adapter or any pc cards that I have, but when I use my dell laptop with internal wireless card it gets a very good signal. There are only two outer walls(one made of wood siding the other has vinyl siding) between the computer and the router. The usb adapter does work if i am near the router and I have picked up a signal with the adapter in the garage, but it only last for a few minutes and then goes away. The computer can see the signal but it won't connect. Any ideas?