Really unstable CPU fan RPM

[cjtyra]

Hello, I am having weird issues with the CPU fan RPM.
If I'm not doing anything heavy with computer the cpu fan looks normal like this:

Stable 32 celsius, nothing special..

If I load the computer with for instance BF3 fan rpm will look like this:

Far right is how the rest of the day will look untill I restart the computer.
up down, up down up down.. and so on. still at about 32 celsiis

I have i5 3570k and Asrock z77 extreme4
Had this problem for years but getting really tired of it.

Original, not clocked, Did bios reset, updated it, Format C:, blew out dust from inside
Would really appreciate suggestions, what more to do?
thank you.

 

[Calvin7]

The fan that comes with the CPU is a 4-Pin PWM Fan. A pulse-width modulation (PWM) input signal gives the ability to adjust the rotation speed on the fly without changing the input voltage delivered to the cooling fan.

Consult your motherboard manual for BIOS options on changing PWM settings.

 

[cjtyra]

In UEFI I can choose from either full on or automatic in fan settings menu. Its on auto with lowest most silent level possible.
Can't unfortunatly see there is nothing more that can be changed..
Everything is so quiet and calm untill CPU is loaded with something heavy for a short while, then fan is getting nervous when it is supposed to be cool and stable again.
Could fan be defect and reading the temp totaly confusing?
Is it possible PSU has something to do with this? just a thougt..