There's a current discussion going on over at Audiokarma about the jerky auto-return I've noticed on a PL-518 I just acquired 3 days ago. I haven't explored any further yet but the response on that issue, from a well respected member who has gone through 6 or 7 of this series of table is that:
1. all had a jerky return
2. lubrication of the return cam or slide had no effect.
3. suspects that the rubbery strip on the tonearm lift is 'grabbing' the tonearm as it slides across.
4. lubricating the strip isn't the solution as it needs some 'grab' to counteract the anti-skate when cueing the tonearm at any point across the record.
5. final assessment is ... you'll have to live with the jerky movement.
So, as I said, I haven't explored this further ... but it makes sense to me.