Check the fuel screen in the carburetor. It's a prime candidate for clogging and causing reduced power due to restricted fuel flow. Also make sure the engine's getting a good flow of cooling air. Cowled engines need to have the air flow between their fins. Many times, cooling air doesn't go through the fins, but goes from the air inlet directly to the air outlet. If you can see the air outlet when you look into the air inlet, and your view isn't obstructed much by the engine, it's likely that the air is bypassing the engine.

If all else fails, take the engine out of the model, put it on a test fixture and see if it will run there. If it will go "all day" with no problems, then it's not an engine problem, but something in the entire airframe/engine/fuel system combination.