Screen goes black, but after pressing the power button twice, it works

Hi guys,

first of all thank you for reading my question. I replaced the screen and the battery of my iPhone 6. When i first started my phone, the apple logo would blink, but not the entire apple logo, only some parts of the logo. But my phone restarted and the screen would just look fine. But after 5 minutes, the screen showed some really tiny green, blue or red lines. After that i powered my phone off and checked the flex cables, but they looked okay.

so i restarted my phone again, the apple logo would again blink but the phone started again. Now the tiny green, blue or red lines dissapeared but i noticed after a few minutes that my screen goes blank. I see that the light of the screen is still on and when i press the power button, the display would go really black and when i press the power or the home button the screen is normal again and i can do all the things i want to do. But after some time the same thing happens again.

Also i noticed that my light sensor is not working, when i activate auto lightning nothing happens, the screen brightness stays the same.

I bought the screen( with camera and so on) and the battery from ifixit.

i would really appreciate some help.

Thanks again for reading my post