we use our Mule fxt 820 to plow snow. It has been fine of the last two years. We have it serviced every spring. This year it started to cough and cug on the uphill part of the plowing. We would let it cool down then it would seem OK for a while. There is no way to get it off the mountain in the winter, so we started trouble shooting ourselves. We found that the vacuum holes had failed, so we replaced them. then we replaced the fuel filter. Those things did the trick It is running fine now. We are disappointed that these things were not a part of the serve last spring, the service manual doesn't include these items as standard Maintenace, which I think is odd. By the way, we purchased the Service manual, which was very expensive. I used to write technical manuals before I retired. This manual it probably one of the worse I've seen., the layout is terrible. References between sections don't include page numbers and the titles are sketchy. Kawaski hire a professional writer.
Anyway, if you are having problems with power or cugging, try the vacuum hoses and fuel filter.