Speed of the Core

Hi,
I have seen the speed of the core on my friend is fluctuating with great difference!
Why is it so? Is this normal?
He has core 2 duo 7200.
Any idea?

I don't think you have to worry about such things its perfectly normal!
Core works according to the commands if the task is complete it comes down its the technology!

For further information please have a look at THIS link!
I hope this helps!