Lurching is not a good thing, I am going out on a limb and say the drive motor brushes are stuck in the holder or brushes have no spring tension due to being worn out. If it has been lurching long time then a possible commutator stone my temporarily fix or even fix the arched out bars. The unit will run with a brush missing from holder and that is where you do your dressing from. Let us know what you find.
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