That sounds pretty weird... Is anything still writing log entries after the computer locks up, but before shutdown? That would mean that your PC doesn't freeze up completely.
If you've really tried everything and the computer doesn't respond to any input except the power button, then I'm not sure what could be causing this. A GPU driver crash would normally be recorded in the event log, so it probably isn't that.
Maybe someone who knows more about the inner workings of FA can help you out
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