the '14 models had problems at elevation,but the '15 and on models have an o2 sensor added take took care of that problem. It sounds like something isn't right, have you tried rolling it in neutral to see if the brakes are hanging up or out of adjustment as @swsebek suggested? How well does it start when cold? If all of that is ok, I'd do the first service on it including checking the valves then see what happens.