>That is why I think that maybe the sensor is the problem... Is this >common after 3 years of use?
I would not call it common but like anything else, electronics can and do fail as it only takes one bad solder joint or component to fail.
>If the sensor is the problem, is it better to buy new one or >to change camera?
Since it is out of warranty, you might want to call one of the Nikon Authorized Service Stations and ask them for a ballpark estimate or price range for the repair. Again it could be the sensor that is bad or maybe just a loose connection. Then base your decision on that information. Good Luck and Enjoy your Nikons!