Battery Low [Q] Is There A Patch to Fix "Wi-Fi Unstable" Problem??

Hello

My Name is Misaki, This is Probably my 4th Post in Total, I Just Recently Bought my Samsung Galaxy SIII (GT-I9300) [2 Weeks ago]
It came with the Android 4.1.x Firmware and then I decided to Update it to the Latest Android 4.3 and The Look is Amazing!!

But as I wanted to Connect to my Wi-Fi router after heading out like 5 Meters away from the Router It says Wi-Fi Unstable!
Yet on my Previous Firm even 30 Meters away I would have 2 Bars on the Wi-Fi Icon at Full 128 kb/s...

The Annoying Part is when I'm Downloading a File Before I know it, it would have switched to my Mobile Data and Taking All my Money!
And Yes I switched off the "Auto network Switch" and Now whenever I'm Downloading a File that does not have resume Support I Have to
Restart the Whole Process!!!

I Have Tried Searching for a Solution to This Problem but with no Avail.....
I Beg of You!!! Please is there any Patch to fix this Problem??