I'd say you have 2 problems, first is a bad clutch disk [oily likely, and flywheel glazed], one way to tell is when starting from a stop, rev it a bit more and "slip" the clutch by engaging it as slowly as you can - you'll likely need to replace the clutch and resurface the flywheel ..... the "jerking" at a fairly steady speed may be coughing or misfiring, maybe timing as said above, or it could be the carb
Bill
