Can you check the battery voltage with a multimeter if you know how to use one? when disconnected at full charge, when connected, and when the strange flickering is happening?
There are battery voltage monitoring tools on Windows and Linux which could work but I’d trust multimeter numbers more. Check to see if it’s above by more than 15% rated voltage or below rated voltage by any amount at full charge. Check for any strangeness in the charge curve.
If you see any of those signs your battery is likely busted. Also if your battery looks inflated at all replace it immediately.