I have a 2018 TW450 that runs fine on flat ground but dies out when under load going uphill. Changed the gas filter and spark plug with same results. Letting it set for a few minutes it will run until it hits a incline. Belt is not slipping, but the engine just dies out and doesn't even want to run in neutral. Any ideas as to what I can try is appreciated in advance. Also only run non-ethanol for fuel.