Blown headgasket. Just had the same problem with mine. Went up a hill, started to turn right and the engine choked down until it almost died and you couldn't see because of the white smoke. Then it cleared out and went back to running ok. Took the spark plug out and it only had 30 minutes of run time and looked like 30 years. Changed the headgasket, adjusted the valves and now its like a new one.