PPPoE clients show DNS problem

I have a PPPoE server running on a wireless AP, and clients get internet connection through a wireless-to-ethernet bridge.

But some of my PPPoE clients running Windows XP SP2 cannot open web pages.

It seems to be a DNS problem, since they can ping and open pages through straight IP address but not through web names.

More strange is the fact that this does not happen over all the machines, but only on 3 over 10.

Any suggestions ? PPPoE connection configurations are all identical.

Thanks
Raf