When you say "unstable" what exactly do you mean? If you hold volume down+home button+power to turn the phone on it will/should enter download mode unless you are bricked. Go into download mode on the phone and put odin on your computer, get the stock tar file on your computer and flash it to your phone with odin.
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If i have helped you out donate me a dollar or so if you can. Im trying to. Buy a new laptop since liquid got spilled on mine.