..but a stable connection with same notebook with external wireless adaptor?

Hi. Ok, here's the deal:

Have an HP ZE2308WM laptop that I bought on the cheap from Walmart.

Internal 802.11g wireless adaptor, from Broadcom.

I have another HP laptop which cost a little more, with same wireless internal card.

1)El cheapo laptop's wireless card works great in the room next to the wireless router; the card works AT TIMES very well upstairs in my bedroom, but sometimes drops the connection. It will see the network, but not be able to connect. (WEP enabled, with the right KEY)

2)The expensive laptop's card works fine upstairs, at all times, no intermittent drops.

3)I've installed a D-LINK USB adaptor into el cheapo; this adaptor maintains a strong connection at all time, with no intermittent drops. It works when the internal Broadcom adaptor goes bye bye for awhile.

Can someone turning on a network next door off and on cause interference? Why didn't it happen with the more expensive notebook, even when my neighbor turned on his wireless network? Why didn't it impact the D-LINK external USB adaptor?