I was having this kind of problem earlier.. but since i have started using the "connectivity" feature of "Imran's Broadband Helper Utility".. this problem has got solved for me.
Code:
This is the fragment of log from the utility:-
06/05/2009 03:29:33 > Unable to open : http://www.ubuntu.com
06/05/2009 03:30:24 > Unable to open : http://www.google.com
06/05/2009 03:31:15 > Unable to open : http://www.yahoo.com
06/05/2009 03:31:15 > No Connectivity Since last 3 Minutes - Rebooting Modem.
06/05/2009 03:31:23 > Modem Rebooted Successfully.