WiFi problem I do not understand

[Aleks A.-Lessmann]

Ubuntu 8.10 running Gnome on a Dell XPS M1330, WiFi driver iwl3945.

I can make a connection with an open hotspot with no problems and good
quality.

I need to log in to the server behind the hotspot via browser. Normally
this goes smoothly and with no problems, but once in about a week the
browser can not make any connection at all, giving an "address not
found" error. The server is running as checked with the hotline.

As I am not logged in, I can not access any other sites either.

DHCP works, as I not only have an IP, but also a default route and a
primary DNS.

No firewall is installed on this system. Normally this error happens
after the Dell went into hibernate. 

Rebooting the OS or subsequent hibernate cycles do not chance the
situation. In fact, the situation changes sometime, somehow, but I don't
know why it does in either direction.

Does anybody know where I should look to find what the problem could be?

Thanks in advance
Aleks