[AC Question]

I've turned it off, taken the battery out and left it out for 10 min, removed the SIM card and nothing. Sometimes it will charge and sometimes it won't. Sometimes I'm able to get it working for a few minutes and then it reverts back to the error message. I've already placed it in factory settings.

[B. Diddy]

Welcome to Android Central! So when you power on the phone (by pressing only the Power button, not in combination with any other button), it goes straight to the Download Mode?

[Rukbat]

Sounds as if something happened to the system partition. If it's a hardware failure, you need a new motherboard, but if it's just a totally corrupted ROM, flash the stock ROM back (see [Samsung] How to flash Stock ROM via ODIN). You've already probably lost anything not backed up, so there's no way to recover anything.