Right most likly carbon build up in your combustion chamber. need a good clean. Unfort have to take the engine apart and kero bath it. It could be carbon build up around the valves making them seat not quite right and letting air in at the wrong times. Also fuel can soak into the carbon and ignite later than normal. You will need a engine refurb to do this properly. also could be your throtle body sticking. On top of the engine is tour throtle body. This is the metal cable going from left to right top dead centre of your engine, it goes in to a half circle wheel as such turn the wheel clock wise when engine is not running and see if it is a clean movement no tight bits, just use some WD40 on this to loosen off. if none of these don't work give me a shout.