well.. not a lot of clues.. If you're sure the engine is not getting too hot, it's gotta be something like fuel starvation.
I suppose some electriccal component could be overheating and semi-failing but thats a little unlikely.
5 minutes doesn't say much. 5 minutes of top-speed driving or low speed driving. Or is it always about 5 minutes regardless? If you shut down and let the bike sit for a few minutes does it return to normal?
It won't hurt to check the entire fuel path from the gas cap on the tank all the way to the carburetor bowl and assure everything is free-flowing and clean.