After thorough search, I find out that the actual problem is RAM Memory.
Using CPU-Z, I found out that the System see's RAM as single channel and Windows see's 4,5 GB of RAM as Hardware Reserved, while after 3 - 4 restarts, it returns to dual channel and the size of Hardware Reserved RAM returns to 1GB.
So, my new question is, why is this happening? I have re-flashed bios and have checked if RAM Modules are properly attached to M/B (DIMM/2 and DIMM/4).
What are you using to check the Bus Width and Memory Bandwidth drop? If you're using GPU-Z this behaviour is expected as with ULPS enabled (ULPS explained here) GPU-Z will display the wrong information as the second gpu goes to sleep.
However should you wish to disable ULPS using MSI Afterburner so it will always display the correct information in GPU-Z, then please go ahead and download MSI Afterburner (Afterburner available here) and select Settings>Disable ULPS as in the screenshot below. Restart your pc and you're good to go!
Thank you very much for the reply, but the actual problem is not CPU-Z. Actually, as I said, I,m using TechPowerUp GPU-Z and both CPU-Z and TechPowerUp GPU-Z give the same values. However, the problem is the FPS drop.One time I have 45-50 FPS the other time, I have 20-25. And that's my actual problem. So, I post to you a print of what GPU-Z.
These values appear when I have FPS drop, so if, for example, i start to play League Of Legends, my FPS will be around 20-25. If I restart and Bus Width returns to 128bit and Bandwidth to 30GB/s, FPS return to their initial value, around 45-50 FPS.
What I also found, is that is happening due to RAM. If you see, Memory Size is 512MB from 1024MB that it originally use. So, I search for it and I found that when this happens, my PC sees only Single Channel, not Dual and when I open Task Manager, I saw that the Hardware Reserved Memory was 4,5GB!
So, regarding to your post - which was very helpful and reminded me of the Afterburner - I believe that the actual problem has to do something with RAM.
If you could give me any new suggestion, I would be very grateful.
Thank you, Tasos.