When was the last time you had your cables replaced? They don't last forever, and I'm sure the suggested replacement mileage is < 7000 mi. If it's a cable issue I'm not sure what you could do to diagnose it. You (or or LBS guy) can tighten it, because they tend to stretch out overtime. It's also possible that there's now some sort of imperfection somewhere on the cable. But unless it's on the exposed part, I don't know how you would necessarily know without pulling the cable out of the housing.