Computer won't connect to internet

I'm having some problems with a residential computer that is running Ubuntu 9.10
It has worked fine for years. Client moved to a new home and has Comcast cable internet.
A couple of Comcast techs got him up and running after some head scratching.
It worked fine for a couple weeks but now he can't connect to the internet.

I've tried turning the power and and off on the modem.
I've checked the cable connection between the modem and the computer.
I'm connected through an ethernet cable.

The computer says that a successful network connection is established but no browsers will work.
I've tried Firefox and Opera. Also I get an error when I attempt to download updates.

Any ideas for where I should start troubleshooting this problem?

Comments

aahh... i will try taking the

aahh... i will try taking the battery out of the cable modem for 20 seconds.

http://www.ubuntux.org/cannot-connect-to-internet-help

update: resetting cable modem

for anyone else having this problem.... I was unsure about opening the modem to remove the battery. Was concerned I might break the plastic getting it open. I located a reset button on the back of the modem which could be pressed by inserting a small screwdriver into the inset button.

1.) I unplugged everything from the modem.
2.) plugged back in the cable cord & power cord (did not plug in the computer ethernet cable yet)
3.) hold reset button down while the modem goes through a series of lights.
4.) release reset button when lights stop flickering
5.) plug in ethernet cable

This seemed to have fixed the problem at least temporarily. I had to try resetting the modem several times with different combinations of things plugged in until it seemed that the sequence above worked. one problem that i noticed was the the "cable" light was not staying lit.

If the problem continues I will advise client to contact internet provider for a new modem.

modem changeout

I have had to change out two modems. The problem seems to be the RF end gets flaky so just do this.

The modems appear to have an upset event every few weeks then few days then few hours.