First of all, I'm sorry that you weren't treated well at your shop. Perhaps you should let the owner know??

AS for the problem, I would definitely replace the cables/housing. What I will tell you though is that the read derailleur housing leaving the frame and feeding into the derailleur wears more quickly than any other piece, so replace that OFTEN! NOt only does it affect shifting, it also wears out that rear derailleur quicker as well.