October 2019 I bought a 2013 420 Rancher FPE ES PS. One owner it had 680 miles and 191.1 hours. It run great, but after driving 20 MPH in 4th gear for about 3 to 5 miles it starts surging, skipping (no backing firing). Stop and let it cool off it runs fine for about 15 minutes and starts doing it again. Then it repeats over and over. HELP HELP HELP.