Why does my Android device unlocks to a non-responsive black screen ?

Device : Nexus 5, running a freshly installed Paranoid Android 6.0.3 ROM. Device is empty except for some system and a few Google apps. Here is the loop I'm stuck in: 1. Pressing power button brings me to the lock screen which is just a swipe. Click sound is heard. 2. Doing nothing keeps me at the lock screen. Swiping up to unlock immediately turns the screen black. Screen is unresponsive. 3. Pressing the power button again does nothing, except the click sound is heard again. 4. Back to step 1. Unable to enter into the phone. This behavior is generally observed after like half an hour of phone usage, when phone gets heated even just a little bit. Phone behavior returns to normal in like 1-3 hours if left untouched. Note : I was running Paranoid Android 4.6 since 3 years and this behavior started about a week ago. I installed the version 6.0.3 about a day ago to see if the update would solve the problem but it didn't. I even gave my phone for repair. I was told the power button on the chip has been replaced. But that didn't solve the problem so I gave it back. This time I was told sensor problem was there but here we are again.