Well, thing is it must be a problem with cooling, because Corona can't use the CPU more than its maximum, no matter what the situation. This just means that the empty scene in Corona is using all threads at 100% with no microsecond breaks, whereas other tests actually have some sort of microsecond breaks that probably are too short to show in monitoring software but still give a core a chance to cool a little.
But overall, this just means your CPU is being genuinely used at 100%, which it is intended to do, but cooling is not keeping it cool enough under those conditions.