Scored on a MS029 20" bar for $100. Supposedly only 2 tanks full[Stihl premixed] through it. Seller said it was a present from wife and it quits after heavy use, took it to dealer and they said nothing wrong. He tried it again and same thing happened, so he wanted to get rid of it. I checked cylinder and it is like new. Haven't had a chance to try it out yet but it is in like new condition. What is the most likely thing that could be the reason it dies when hot?