MacBook Screen Flickers and Shuts Down Randomly After Battery Change

My MB Pro Retina late 2012 served me very well for the last couple of years, until the battery was virtually dead and I replaced it with an iFixit battery.

After the replacement, I calibrated the battery as suggested from iFixit and everything was working well for the first cycle of the battery. I erased the hard drive, installed Snow Leopard, updated to El Capitan and eventually updated to Catalina 10.15.6. But then some problems started occurring, like not waking up after sleep mode. After using it a little more, I experienced some random shut downs where the screen would just turn black and I had to restart the MB. After using it for some more hours, a weird glitch started happening: after half an hour using the laptop, the screen would start flickering weird in colourful horizontal lines divided into columns across the screen. This would go one for about ten seconds and then the MB shuts down.

I tried resetting NVRAM/PRAM and SMC but this didn’t solve the Issue.

I tried to figure out, what could be the problem, but the more I want to use the MB, the faster this glitch and shut down occurs. Sometimes it happens right after trying to log in, sometimes it would run for a few minutes.

It happens when the charger is plugged in, as well as wenn it’s running on battery. When the power cable is plugged in and it starts with the flickering, and I unplug the cable, the MB would shut down immediately.

Also, I can’t enter the recovery mode, the glitch and shut down always occurs while the loading bar is still loading.

I never had these kinds of problems before the battery replacement. Can it be a problem with the battery? Or did I break something during the installation of the battery?

Anyone have an Idea what the problem could be?

Thanks!