anybody with experience on diahatsu/briggs diesel on scag sabre tooth tiger? with mower engaged, machine bogs downs in all but the shortest grass, or mowing on any incline. have replaced fuel filter, belt and deck spring (broke 2 coils off the end), blades sharp and straight, all greasing done religiously. all spindles turn freely, gearbox is ok as well. unit only has 269 hours - 3 mowing seasons. runs/sounds fine without mower engaged. does not smoke, or run rough, just slows down in grass it normally destroys at top speed. it's in the shop now, but dealer is not sure what is the cause - they have more than 30 of these out in the field and have not seen this problem before.