Well if you had a custom ROM running before and that was working then it is likely that it will still work. If it doesn't then you can always flash to stock in Odin again. Its very unlikely that you'd end up worse off since youll just have the same issue on a custom rom if it doesn't fix it. If it doesn't fix it then it is mostly likely related to the baseband because that is one of the few things that remains the same between a custom ROM and stock since you should be wiping data and cache when you flash a rom and it completely rewrites your system. It may not fix the issue but its definitely worth a try and if you decided not to upgrade to the 4.3 bootloader then there's not much need to be stock in my opinion. Stock offers very few if any advantages over custom roms. If you need high stability then just use a ROM that is based on the stock ROM, they are almost always bug free because they only add things like root, deodex, reboot menu, zipalign, performance tweaks, etc. They don't have to deal with drivers or source code like on AOSP roms so they are much easier to make. It may be worth a factory reset now on the stock ROM too if you didn't do it after the Odin flash. Sometimes old data from another ROM can cause issues.
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