I recently purchased a 2008 700 xtx (1100 miles). I took it to Moab a couple of weeks ago and it began sputtering at low speeds. I changed the plug and it did it at low and high speeds. 20 to 35 mph would run fine. I checked the air filter and it appeared overly dirty. I cleaned the filter and put on a light coat of oil. Still sputtering. What do I need to check next?