Sounds like the battery is failing. I had a similar issue and a battery replacement did the trick.Yeah my phone is a 6S and I plan on keeping it until it dies... It works great today. Meanwhile my Android phone before it could not upgrade past 4.4 and barely made it two years before it became impossible to use. If I used it while being charged, it would often lose battery and it could die at anything below 50% battery when not plugged in.