Hi
Try going to Settings > General > Reset > Reset All Settings and see if that makes a difference. You will need to set up some bits and pieces again after, but it won’t delete any files or photos.
Some owners found it was caused by an old backup. If you are restoring from a backup you made with an older iPhone, that could be the problem. Try setting the iPhone 6 up as new, factory reset it, and then put everything on it manually, and you might find that the problem is gone.
If it persists, then you may have a hardware fault and you should contact Apple or head into an Apple Store to get it looked at. They should replace it for free if there’s a problem.
01-27-2015 02:14 AM