I had an old Thunder Tiger do that and turned out it was the carb barrel sliding back and forth altering the size of the opening in the carb throat. The spring that sat under the screw holding the carb barrel in was worn out and let it slide back and forth. For a short term fix, I bent my throttle wire outward a good bit and installed it back in the throttle arm. That put outward pressure on the barrel, held it in one place and stopped the surging. I didn't have to worry about a long term fix because I was flying in too much wind one day, augured the plane in and smashed the engine to smithereens.
You might try to slide your carb barrel back and forth and see if it moves. You wouldn't think so on a NEW engine, but maybe....