Singer sewing machine. Have dusted, oiled adjusted over and over. May work OK a bit, then goes back to breaking thread so back looks like hair. Seems like thread getting caught in shuttle?
Sounds like there could be a burr or nick somewhere that is catching the thread and shredding it. Change the needle. Wrap your index finger with some fine fabric (old hosiery, chiffon, silk) and run it around inside the machine around the bobbin/shuttle, feed dogs, needleplate, presser foot. If there is a burr, you will probably find it. If it is minor, it can be smoothed with a fine emery board or fine sand paper. If it is major, you may need to replace the part.
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