As has already been mentioned, the type of surging you are talking about is usually from too lean of a air/fuel ratio during cruise and part throttle. !st I would check ignition timing to see if it is too far advanced at the rpm you are experiencing this or if at all levels then it should be retarted all across the board.
What is your ignition advance curve from begining to end? At what rpm does it start advancing and at what rpm is it fully advanced? If you are using a vacuum advance canister be sure the hose is connected to a ported vacuum outlet and not to a full time vacuum source. That can really cause response problems.
Ed