rev counter faulty?

Hi guys, just having a problem at the moment where when my bike is running for about a couple of minutes the revs will sit accurate, but when I ride down the road it will just start going all over the place? does anyone know what could be the problem?

It could be because something is slipping such as your drive chain or the sprockets, make sure that your drive chain isn't too loose and that your sprockets aren't too worn.