On board / internal display works intermittently / randomly
I see you tried resetting the NVRAM (PRAM), but have you tried resetting your SMC?

For your model, the steps to do this are:


Shut down your computer
Keep the MagSafe adapter (power cable) plugged in.
Press at the same time shiftoptioncontrol (on the left side of the keyboard) and the power button
Let go
Turn your computer back on with the power button.


Let us know if this makes any difference.