Hello TengoKGB,
Thanks for the post in Apple Support Communities. I understand your Mac seems to be showing some flickering lines when you start it up.
Is this only happening during the startup process, or do you find it sometimes happens while using the Mac as well?
As a start, I recommend that you reset your NVRAM as shown here: Reset NVRAM or PRAM on your Mac
If the issue persists, try booting your Mac into macOS Recovery mode and keep an eye on it to see if the lines appear while macOS Recovery loads, or after. To access macOS Recovery, turn your Mac off and immediately press and hold Command + R until you see a globe: About macOS Recovery. After testing, just click Apple menu () > Restart to get back to normal.
Take care.