Why Does My Internet Keeps Disconnecting After A Short Period Of Time?

I have no idea why but my intenet keeps disconnecting after about an hour or so, i thought it was the modem(cable linksys gc200) i bought a new modem (motorola surfboard 5100) no luck, I am connected by lan ,I have tried usb that didnt work either. Also i tried the latest firefox and i have crome and the latest internet explorer all of them do the same thing, im on a vista and i do updates regularly. my antivirus is avg(updated) and malwarebytes i run them both on a regular basis.
What could be causing this i called my provider they say everythings fine, the internet connections icon on the lower right has the globe and the modem has all the right lights on but the internet still doesnt work, when i do open my internet and try surfing the web it just goes to the “this page cant be displayed” i click on diagnose internet connections tab and it will run that, then it would pop up a window that says windows found a problem but cant repair automatically. i have tried reseting my modem when the intenet stops working and nothing helps, ive also checked windows firewall for any problems there doesnt seem to be any. Any help would be greatly appreciated.