My 440 is having a little trouble. Sometimes when I finish a cut and let go of the throttle it'll die. It's not consistent only doing it every once in a while. When it happens I can hear a change in sound and if I'm quick enough to hit the throttle I can even it out and then it'll idle fine. If it dies 1 or 2 pulls and it's back up and running. I have checked the plug, cleaned air filter, changed fuel filter and re tuned. What do I look at next?