I would doubt a bad valve seat as that would be present throughout the rpm range, it's more likely that Ian's correct about a vaccuum leak...tops of the carbs, manifold rubbers, or squirrel condoms all come to mind from what you've described.
A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort. Herm Albright (1876 - 1944)
