DAB: When I suggested replacing the cables I should have been more specific that you should change both the inner wires and the cable housings. Don't skimp on quality, get die-drawn stainless wires and lined housings. Prep and install them properly with the correct ferrules and pay special attention to the loop which enters the rear derailleur. If as suggested by Mr. Beans you have fraying inside your shifters you will be glad you replaced them before they broke rather than after.