Chosen Solution

iPhone goes dim randomly and then completely turns off and restarts. Even when the phone is locked and the screen is off it will sometimes restart

Let’s start with this How to Force Restart an iPhone 6 Plus Next thing I would do is try a new battery. It’s possible it has issues keeping power and being able to do things iPhone 6 Battery iPhone 6 Battery Replacement

Have you found the solution? My iPhone 6 has same symptoms.