A friend had some still alive ash trees taken down by crane. They were all bucked up, just lying by their driveway for me to cut and take home. I haven't had as many cutting issues since my rookie year cutting. I brought 2 Echo CS-400 saws w 18" bars. (Both chains done by a shop, one with a new Oregon bar - Due to loaning one to a friend. Won't do that again...) One soon started cutting crooked, the other quickly dulled, but was able to finish cutting enough to get the wood home. I put a new Oregon chain on the new bar. It cut like a champ. Until it suddenly stopped cutting. Sparks observed coming from inside the cut. I noticed the black center core of the tree on this cut. (See pics) I grabbed my old McCullough. Same story - Cut really well until it stopped throwing big chips. Instantly stopped. I know I did not hit anything on the ground. Have not seen any nails, etc in the trees. Thoughts?