My brother’s iPhone 5s (16Gb, approx 2,5 years old) had a problem with it shutting off at random, after which it would boot after some time. When it would boot, it wouldn’t require the sim unlock, it would just be as if nothing happened. We diagnosed it to be the battery, since that seemed to be a common problem/solution we found on the internet. And after replacing the battery this morning the phone indeed worked like normal. But after using it for a bit it started showing similar behaviour again.

Does anybody have any experience with such behaviour?

Alfa-Romeo Tipo 33 Stradale for your time: