Core Speed Problem

[larzlo]

In the bios I currently have my i5 4670k overclocked to 4.2 GHz at 1.2V. I use Intel Stress Test and it passes but the problem is on cpu-z I notice that my core speed drops down to 3.4 GHz at times, the stock cpu speed. I'm wondering if this is a problem or if it's normal.

 

[schmuckley]

That's perfectly normal..
You probably overclocked "turbo core"