Well.... yesterday I have tried it couple times again and I have finally managed to get this working.
I tried almost everything, but this one way works for me, I dont know why, but it does.
And by the way, I actually cleared the correct bits, the problem isn't here.
First, I restart my computer. If I don't restart it and clear that bits, my laptop freezes.
Then I open-up AMD overdrive. In the Clock/Voltage tab click "Turbo Core Control" and make sure its enabled.
If I haven't restarted my computer, the "Turbo Core Control" option would be grey and not available to click/activate.
Then I click the green sphere at the top of the AMD OD GUI, then red circle around it appears. After it my clock finally go over the 2,5 GHz.
...but it still randomly drops between 2,4 and 3,5.
If I do all of these steps, I am able to change those bits without any freeze and its finally locked on 3,5. (Its actually 2 cores on 3,2 and 2 cores on 3,5... no idea why as well ->according to AMD OD)
I don't get it, why does it work this way.
Now, with slightly OCed 8970HD, I can finally run BF3 on ultra at 45-65 FPS!! Yay! Other games run much better too of course.
Thanks again @Khenglish for your guide and all the others caring about people like me!