Screen glitching, white bars showing, no screen cracks

I own a iPhone 6+, which have some problems. I switched to android when it started happening, but i want to find out what the problem really is. So, sometimes my iPhone works just fine, with no problems. But then the screen suddenly wont respond, and when i the slam it against an object a few times it starts to work again. Sometimes its like a white bar gets dragged down from the top, blinking and going up and down. This started happening after i got my screen replaced. I took it to another shop, and they said there could be something wrong with the motherboard, and the couldn't promise a replacement of the screen would fix this problem.

If anyone could help me i would appreciate it!