Another piece of information: all has been going fine. Then I sat and played Hill Climb Racing for about an hour. Then - black screen. No signs of excess heat or anything similar, just nothing. Eventually the boot logo reappeared, but I could not dicover what state the tablet was in. So I plugged it back in to the charger and forgot about it until now.
Hit the power button, I have a white screen with a fade-out border. Then a home screen, still with traces of this border. Then that has faded out to normal, and the tablet is once again seemingly normal.
Some of this seems familiar from reading the thread, the odd border for example.
Will probably, at this point, go back to 16 for a while and then come back to 18 in the future. Keep up the great work!