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Sounds like one of the timing sensors is dirty - possibly from paper dust.
I don't know if they use the "flag" type sensors with the actuator arm, or a optical sensor.
Look for either a plastic "arm" sticking up or a small square sensor with two eyeballs on it right where the lead edge of the paper is stopping, and make sure theres no dust, 3-ring binder punchouts, etc around it.