Graphic issues with animations and problem at turning on

It won't turn on without the charger if I'm not holding power for almost 15 seconds, and when the charger it's plugged in I have to press the power button. In both cases, computer enters in a loop and turns on and off three times and then it turns on correctly.

• When it's booting with Apple logo, grey background is at a slightly different color while loading bar it's showing and after I enter my password, some horizontal lines appear. Lots of time for loading bar to finish and Finder to load after password.
• Sometimes Finder bar is black but then it goes normal
• Dock lacks that "transparency", it has a white background.
• Any animation (e.g 'Downloads' folder animation at dock) that happens has weird lag and the computer goes so slow.
• Some programs are unusable because of those weird black lines like in startup (e.g Google Chrome, Spotify while playing a song, etc.)
• Computer turns off when display sleeps WITHOUT charger
• Spinning wheel SO often

It seems to be a GPU problem, I tried connecting it to an external monitor and nothing changes. Tried to reset NVRAM and SMC, but it's kinda impossible since it doesn't turn on immediately. Also made an Apple Hardware Test, both short and long scans, nothing detected. Booting in Safe Mode, same problem.

The weird thing is that it happened twice that I turned on and EVERYTHING WAS WORKING GREAT, NO ISSUES. But when restarted it, problems again.

I turned off my computer and the next day was like this, I didn't move it from my desk. I've never spilled anything or hit my MacBook.

Thanks in advance!