OK it's RPM related which points to the carbs...
EDIT: After thinking about it for a while, it could be related to a plug or two being weak. I'm thinking try a set of iridium NGK plugs first before you start tearing into the carbs just to prove it is indeed a carb adjustment issue first. But, before you do that, dump a whole can of seafoam into a full tank (double dosage) and drive it like you stole it. If there is a lot of crud to be burned off, it can and will foul the plugs even worse so you may as well totally wreck the old plugs...