Great news!
FWIW, the LBS is right, the OE rear derailleur is marginal at best, I replaced it with this one, https://www.amazon.com/gp/product/B003OWPRLI/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1
Now it shifts quickly, precisely, and silently without any need to fiddle with the levers...