My display seems to fail randomly, even after changing LCD. What could it be?

So here's my issue. Yesterday, my phone started to give me a while screen full of dead pixels (pinkish color) randomly. I would turn off the phone, turn it back on and after 2-3 minutes it would crash again. Last year I dropped it and cracked the screen, so to me it was related to that. The phone works fine other than that (music plays when display fails. I did wipe it completely and reinstalled Kit Kat, it solved the issue temporarily but after 30 minutes it started again.

Today I decided to go check out a phone repair shop and ended getting my phone fixed by them. What this consisted of is a replacing the display and plastic frame. I picked up the phone and everything was perfect, new screen and display which looks and feels like new.

Now it's been about 5 hours and the phone started acting strange again. Now it's a lot less worse than before, but I still get the full screen of dead pixels. The display will go on and off (I guess it's trying to reinitialize or something) but always fails. So I end up rebooting the phone for it to work again.

The 2 times it happened after getting the phone fixed it was when I put it in my pocket to go from my work to my car, and then from my car to my home. So I don't know if it has anything to do with being in my pocket, that's why I came here.

So could anyone could shed some light on my issue here? Is my phone already dying? It's only been 13 months :( I'd hate to have spent for a new display for basically nothing.

Thank you!

When you dropped it, the motherboard probably cracked. The crack gets worse as time goes on.

Will it pay to replace the motherboard? If it's a $700 phone and you can get a new motherboard for $150, sure. If it's a $150 phone and a new motherboard is $100, probably not.