iPhone 6: Unexpected Restart

Hello.

I have recently changed my iPhone 6 128gb because its screen bent. Apple Store gave my a new iPhone 6 128gb.

My product model is A1586.

Since I got the phone and turned it on, it keeps unexpectedly restarting.

The iPhone freezes for 1 second into a grey screen (sometimes red) with a pixel line in the middle of it and then restarts.

It is getting impossible to use the phone. Those restart have been happening more frequently.

I already tried everything:

• Force reseting
• Restore in Safe Mode

I suspect it is something related to the battery since the battery is not charging very well when those restarts are happening.

I really do not know what to do.

Any ideas?

Thank you very much!

Regards.