i have a 07 420 rancher 4×4 manual shift…. its started shifting on its own in the mud.. it doesnt do it on pavement or dirt but when the tires are in mud as i reach the top of my RPM range it will shift from first to second, then if i get it up enough second to third… it never shifts down nor acts up out of the mud…. the bike doesnt have ES and never did…. any time i try to google it all i can find are ES shifting problems… beating my head off a wall here any directions where to look/ ideas what it could be are appreciated