Hi Ive had this issue, a tough one to problem solve, perfect runner on the ground on thrust but if you were to continue full thrust for 1 minute above power loss around 25% occurred until back to idle & then back to full & same again. Turned out to be the exhaust outlet & exhaust box (muffler) heavy build up of carbon & deteriorated exhaust insulation. So the head was removed & decoked at the exhaust port using a wire snake & same at the exhaust muffler followed by new insulation wool. The problem is now resolved.
