firefox 3.5 on vista stops responding

im using vista on my laptop all my updates are complete, using avg free, zonealarm and windows defender. the web browser firefox 3.5 works fine for x amount of time then suddenly cannot find any webpage on the bottom left hand corner page says done, right hand side shows connection error and a message saying cannot find page click here to retry, in command prompt if i ping 127.0.0.1 and www.yahoo.co.uk they both pass and do not connection time out. the icon on the taskbar that shows connected has the two monitors with the ball and shows connected. only way to get back on the internet is to reboot pc, this is the same for firebox safemode, explorer and explorer safemode

forgot to mention that when the problem happens i can still get email and use msn, everything else works ok apart from webpages.
oh yes and i have to force the pc to shut down as it wont once its had problems
also i have wiped the pc with a fresh full reinstall, redid all updates ect.