Could be anything from dirt and water in the carburetor to a valve getting hot and coming off the seat and causing the engine to lose compression. Wondering which way you were going on the inclines? Sideways would cause fuel and oil starvation on a prolonged cut and cause the engine to run w/o oil. Eventually that could cause a crank and rod to go out. Then the engine is junkpile bound. I'd check for clean fuel all the way from the tank, fuel line, carburetor intake fitting, float bowl and pickup tube. Only way to check for valve clearances is to take the valve cover off and visually inspect and check with feeler gauges.