sounds like a bad case of clutch slip, check the pressure plate. Im believing that is was replaced with a NEW GENUINE PART. Secondly check the rubber pipe going to the clutch slave cylinder it can act like a slow release valve when it swells up with age. Its supposed to be replaced at about the same time brake rubber pipes are replaced.
When buttoning it up, use a coat of heavy moly grease or genuine CV joint grease on the input shaft splines of the gearbox, This will ensure the clutch plate is free to slide on it. A very very thin application of the same grease on the starter ring gear also helps elongate starter life and prevents rusting too.