GTI19300 Wifi problem

Hi folks.
I will start simply, my wifi only works when im very close to my router.
When im at certain distance, it appears an "authentication problem", that suddenly disappears if i get close enough to the router.
I already have searched for solutions, nothing works. At least nothing that i know how to do.
My knowledge with android sistem is minimal.
The problem isn't my router, all other smartphones works fine, just my own is having this problem.
What i have tested and had negative results.:
- *#0011# and set the wifi power saving off ( something like that)
- WPS connection works, but when i go 5 steps away from my router i cant connect to anything. It appears that wifi is connected, but doesnt works.
My GS3 has nothing "special", no ROMs, kernel or mods. Its running on 4.3 Android.
I really do not understand about android. Have seen in some posts that people changed their version and the problem was solved.
Should i try to restore the GS3? Havent tryied yet.
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Posts that i readed.
http://forum.xda-developers.com/gala...oblem-t2662574
http://forum.xda-developers.com/gala...-wifi-t1779535
http://forum.xda-developers.com/gala...ation-t1805518
http://forum.xda-developers.com/show....php?t=2098278
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