iPhone screen turned black and unresponsive when lock or in sleep

Heres what i tried already:

-hardrestart(holding sleepwake and home button)

- holding all buttons including volume buttons

- it turned back on but still has the same problem whenever i locked it or when i left it to sleep(autolock:1min) so i had to hardrestart to turn it on again

- i reset the phone to a new phone. Still the same prob

- update to latest ios.

- when it blackscreen again i tried to toogle the silent button to see if the lcd is respondsible for it... Still unresponsive so its not the lcd

- tried to change the battery.. Same prob

To use it again:

-I had to hardrestart to turn it back on

-and turn off the auto-lock

is it a hardware or a software problem?????

Please help...