Hi and thank you for using Just Answer.
Unfortunately these trimmers don't have a long life. The motors in them just can't handle extensive use. What you are experiencing is the end of life for that motor and for what it cost to replace, you are better off getting a new one. The brushes are either worn in which it should start to make some noise, but I would guess that the armature got hot 1 to many times and the wire in the coil has started to separate. When it starts to do this, it will give you the symptoms you are having. I'm sorry I can't give you better news. I wish you the best