Yesterday I turned one of the boards back to 1.2GHz (3pm), and it died during the night (2am). The 920MHz is still working. The room temperature didn't go over 26C (the thermometer is by the boards).
Status update:
One of the boards failed at 920MHz @ 60% temperature levels. I blamed the power supply and switched to make sure the *other* board would fail as well at 920MHz, which it did. So I retired that power supply and am using a third one, all cheap (all I have here).
I'm not sure how it managed to run for two full days before, but it could be due to the temperature of the room that has increased these last two days and the power supply itself is overheating.