my iphone 8 black out issues

my iphone 8 blacks out most of the times, somtimes it reboots on its own and sometimes i have to press volume up and down key and then press the button on the right hand side for 10 seconds

and sometimes the phone does not restart with the above steps also, it takes few hours or sometimes a day to restart, it just dies off.....

what is the issues

with any app? can we check what could be causing this

or the phone itself has an issue