ok so I installed my FP2 from scratch, as AnotherElk described here. (One more time: thanks for that!!)
I tried the three latest FPOS versions 19.08.1, 19.05.3 and 19.02.1 (currently running).
I can watch videos now, but the artifacts still occur the same way. And there are still waaay too many random reboots, clearly without overheating. Additionally, my phone lately stopped responding when the alarmclock went off. It just keeps ringing, touch and buttons out of work and the only way to turn the alarm off is to remove the battery.
I didn’t install many apps, deactivated almost all googleapps as I’m not using them and I periodically reboot my phone to keep the phone from running too many apps in the background.
Nothing I tried really helped. As I said, I really need a reliable phone when I’m out in the mountains and my FP2 clearly isn’t anymore. So I came to this point to soon say goodbye to my FP2.