If there is carbon build-up or oxidation on the drum OR motor brushes, operation will be intermittent. Same goes for the pickup shoes (rollers.) Spray some no-residue electrical contact cleaner in the rollers where they turn on their axles. Also on the e-unit and motor brushes. Give the vapors some time to evaporate (they're flammable!) and give it another run.
Running it may help to burnish the contact surfaces; operation may get more reliable as it runs.