I have an '06 sportsman500 ho, it was running fine, shut it off went to start and it turned over and over. towed it home started up again. next time hard to start, next time it wouldn't start.I have spark and fuel. changed fuel and air filters,spark plug, and battery no better. adjusted intake valves to .006 started right up! rode it twice started to act up third time it wont start!!! PLEASE HELP!!