I have a similar issue.
I too have a Samsung NP350V5C running Windows 8. Up until a few days ago, all was fine. Now, if I boot up the system, my function keys work as expected for about 15 minutes. Then, it is as if I am constantly holding the FN button down.
So for instance, pressing F5 will no longer refresh the browser screen, instead it will disable the mousepad. And pressing F2 and F3 causes the volume to increase or decrease.
It really is annoying. The only way around it I have found so far is to reboot the PC, and then it is fine for another 15 minutes.
Also, it isn't a browser issue. The PC can have no apps running and the problem will still occur.
I am sorry I don't offer a solution, but at least you now know you are not alone!