You should be replacing the plugs every year if you summerized the sled using the computer. That fouls the plugs and they don't really clean up even riding hard. Do not remember the gap but typically the NGK's come with the correct gap but you should always double check. Owner's manual will have that info. I have a 2015 154x with 5800 miles so far and always put in new plugs every year along with cleaning the rav valves at least every year but sometimes twice a year depending on miles ridden. Fuel filter every second year depending on fuel and I only run 91 non ethanol and use BG Super Charge every time which keeps the carbon down, fuel system clean, and moisture out of the fuel. Sled runs flawlessly until either plugs are bad or valves get sticky. Always blow clutches out every ride and disassemble every 500 miles to clean and inspect. Only had to replace the arms and ramps once so far but always the spring every year. My bet is the plugs will cure your problem.