iPhone 6s shuts down

iPhone 6s shuts down fully when entering sleep mode on its own, if sleep button depressed, screen turns off normally and will turn back on as normal, but if left for any given time the phone shuts down.

also

when phone shuts down have to hold the power and home button for 30 seconds to start up again, it will not power up with just power button pressed.

Have replaced battery ok, have reinstalled OS ok, all other programs working ok when phone is on.

If i disable the screen lock phone will stay on until battery is flat.
😮
any ideas ?