[Q] Need help with flashing roms Galaxy s3

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http://forum.xda-developers.com – I tried flashing two different roms (synergy and chemistry) on my sprint galaxy s3. It turns on but then wont go past the "galaxy s III" logo. I just restored my backup and all is well but I really want to get multi window on this thing so I could really use some help as to what is not letting me boot up all the way. The last thing i messed around with rooting was my DroidX and with that it really mattered a lot which version of android you were flashing from. I have 4.1.2 the most recent update. Is that why these roms are not working? Any help would be much appreciated I am very exc (HowTos)