I have a Gigabyte GA-P67A-D3-B3 motherboard, KINGSTON 2x2GB DDR3 1600 MHz RAM and an i5 2500 CPU.
So my problem is/was that the memory and MB are certified to work @ 1600 MHz but CPU-Z shows that it runs @ 1333. I changed it in BIOS to 1600 but is it safe (I had not any problems)?
And the CPU Frequency goes up to 3600 GHz when ram is set @ 1333 and to 3400GHz when RAM is @ 1600 MHz. (Is that normal)
The CPU frequency should not change when you change the RAM frequency. Is it possibly your CPU is changing speed based on load as most newer CPUs now do? Under light load the CPU voltage and speed lower and then increase with load.
Run Memtest86+ to confirm the RAM is stable then you can use OCCT, Prime 95 or Linpack to stability check the entire PC.