See if you can dig up a 3 part stock firmware to flash (assuming you haven't already, a little vague about exactly what you have flashed, I also presume you're talking about an I9100 ?). Could well be a case of corrupted NAND, this is sometimes the case where you seem to get a successful flash but bootloop won't go away & you're still stuck with only download mode. Fix for that is a motherboard replacement, but I wouldn't be calling it just yet.
If you can't find a 3 part firmware (or if you've already tried one), just get hold of as many stock roms (from Samfirmware/AndroidFirmwares.net/Samsung-Updates.com) as you can & flash away; sometimes these cases are fixed by perseverance (the first 10 flashes don't work, but for whatever strange reason the 11th works, for example).
Search for Hopper8's 'Odin troubleshooting' thread & try all the stuff in that thread, might be something really small/obvious that's causing problems here.
If it's important enough to cause you grief if it's lost, back it up. No excuses
Trolling for thanks is lame
Does Samsung's auto-brightness implementation crap drive you to self-harm ? A solution !
Probably the best thread ever posted on XDA
Failure to search is the leading cause of male impotence on XDA
If you don't know how to recover if things go wrong, what are you doing flashing custom roms ?