My shifter cable for the rear derailleur pops out of its enclosure from inside of the lever. I have attached a picture of the end of a short ride today, in red is the ball of the cable popped out of the green enclosure. Normally I would think it's the tension of the cable but I adjusted it and works and shifts well after adjustment and for the first 30 min of a ride. should I change the cable? Is it the shifter that need changing? It's a Shimano 105 shifter, about 6 years old,13k miles on it. Cable is about 1 year old.