losing connection wireless network card with Thomson 570 modem

Iam using a laptop winXPhome with 2nd edition using a speedtouch 110
wireless card.
WEP on.
Everything is functioning, the communication with the modem and connection
to the internet.
after a few minuts the speedtouch card is disconnecting and the computer is
telling there are other networks available and the network card icon is
blanc. clicking this icon you can see the network card is scanning to other
channels.
Clicking in taskbar on the wireless connnection icon it said reconnect to
the speedtouch modem, after clicking the connection is there again.
The modem is already in the preffered list. The signal quality is excellent
and stable.
Please give me an suggestion about what iam doing wrong and what to do to
make it functioning.

thanks in advance,

Joep