Problem with Livebox (WPA)

I have a problem connecting to my Orange Livebox - which uses a wpa encryption. According to network manager, i am connected fine with good signal and an ip address and everything. When i try to use the internet, it sometimes works fine for about 30 seconds, then dosent work at all, even though it still appears to be connected. I am running 64bit Intrepid with network manager. It is a toshiba satalite with a Peak 11 Wireless Lan USB Adaptor.

I should point out that it works perfectly at my flat, which uses WEP and i can connect to a router which has no encryption, and my other laptop running Vista can connect to the livebox with no problems.

Thanks,

Fred.