On stable at idle but flickering at higher engine speeds can suggest a couple of things, one being a poor connection some where that is affected by engine vibrations another is that the brushes are worn down to the limit, contacting the commutator OK at idle but not at higher revs, at their wear limit, the spring is still applying pressure but the brush ' tail ' prevents full positive contact with the com and the brush tends to skip, for a better word, making intermittent contact.
This normally cant be seen with a conventional DVM but can be seen on a scope.
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