"Forbidden access" to Google newsgroups via Apple Airport

Hi,
I just installed an Aiport access at home connected on one side to my
provider cable-modem (Noos, France) and on other side two Mac computers.
Everything is fine (mail, web) except that, strangely, I can't access to
Google newsgroups :
-> I can reach Google home page but as soon as I click on whatever forum
link, I get a "Forbidden access" page from Google. This effect can be seen
on both Mac computers.
-> When I tried to come back to a direct connection from the Mac computer to
the cable-modem, it is ok, I can access to Google forums.
-> I tried also to set the provider proxy's address without any success.
-> I checked form an other computer with nslookup that DNS addresses that
DHCP gets from provider are ok. But, on the Mac OS X laptop, nslookup fails.
-> I tried to see what is going on by using tcpdump and I noticed that
laptop is trying to do reverse resolution of himself to the Airport without
success (PTR query but no answer from the laptop). What does it mean ?
For reference :
Airport software 5.1
PowerBook G4 1GHz, OS 10.2.6

Regards
Antoine Davous