I only use Aurifil for piecing and love it. Great tip about pulling on the thread tail to see if the needle bends. I do this all the time, sometimes when threading a machine it doesn't set well in the tension disks. If it does bend the needle, re-thread. Now for free motion I use a polyester...I know the quilt police are going to arrest me. I have a very good friend who does professional free motion on domestic machines and long arm. She says it's stronger. I agree speed and movement can cause a problem. I use an embordiary thread I get at my machine dealer and it's beautiful because it has a slight sheen. Good Luck