Take the time to look up exactly what cool and quiet does on an motherboard.
First of all if the cpu gets to say a temp set up on cool and quiet it will throttle back the cpu speed to lower the heat build up. It does not care if you are running a burn in test it will do it regardless if cool and quiet is set up on the board.
That temp drop is cool and quiet kicking in.
Trying to cool the cpu, second of all it will also ramp the speed of the Fan automatically to get the heat away from the heat sink.
Obviously change the setting in the bios on when cool and quiet kicks in.
Or turn cool and quiet off in the bios settings of the board. Manualy set your fan speeds.
It will also stop it from throttling the cpu speed. Or set the trigger in Celsius in the cool and quiet options in the bios settings of the board 60c is ok. Your default setting is probably a bit low so the reason for it all FYI. A trip to the bios me thinks!