Screen not working properly after replacing?

My friend asked me to change the screen of his iPhone 6. I have changed many 4s and 5 screens, but never a 6. I did everything like I always do and it turned on normally. The apple logo stayed for about 5 seconds and after that the screen went black. I checked on it a minute later and it worked normally for maybe 15 seconds and then went black again. It does this all the time now. Works for about 20 seconds and goes black, then I leave it on the table for one minute and it works again for 20 seconds? I have shaked it and tapped it and that does not affect in any way. All contacts and cables are good. I once had the same problem with an iPhone 5, but ithought it was because of a faulty screen.. Please help solve this problem if you know why it's like this!