Yes, it's all rather confusing really, isn't it?
![Shocked :o](http://forum.giga-byte.co.uk/Smileys/default/shocked.gif)
It really does seem to depend on:
The processor
The Multiplier that is set
BIOS
Motherboard
and all this in no particular order and without any real logic.
However, usually when overclocking you wouldn't want the Cool&Quiet feature, as this throttles the processor speed and voltages and the usual practice would be to disable this before overclocking. Maybe that is why it works with a minor O/C but doesn't when you go above a certain point.