iPhone always switches off when it's not being used.

iPhone always switches off when it's not being used.

When I don't use phone, screen is going to sleep (is black) what is expected behaviour, but and after few seconds (usually less than minute), when I try to use the phone, it seems to be switched off. Screen is black, power button doesn't work, I can't feel click from home button. I can only make hard reset (power button and volume down pressed) to bring it back. Hard restart works only once, next time I have to either reconnect battery to turn phone on or wait long time (few hours).

I did some tests and started video recording, for 8 hours screen was active (switched on) and phone didn't switch off at all. I did software restore and it didn't help.

It seems to be software issue but on lower level. Do you have any idea what could be the reason?