Originally Posted by
krems81
take it back to the shop, but one thing to check is ghost shifting sometimes happens when the rear der cable isn't traveling freely in the bottom bracket cable guide (underneath). If its sticky, when heavy torque on the crankset twists the frame (this is natural) the cable moves with the bottom bracket shell. leads to some funky shifting. worth putting a dab of grease in the guide where the cable runs under the bottom bracket.
Good thought ... also check, or have the mechanic check, that the cable is routed correctly and isn't mistakenly next to the guide, not in the guide.