Phone comes on and off

At times when I turn on the phone it turns off immediately or after a while,

and if I try to turn it back on instantly it won’t turn on again before an hour or

more sometimes and sometimes less; Most of the time before it turns off

completely I get a black screen ; During that time I can receive and answer calls because

for a short moment the screen comes back on again. In other words it’s fooling about

and “ When” it works , it would be only for a very short while.