OK, it appears that I can get it into download mode if I run it off of USB power, and pull the battery as soon as I'm able to get into download mode. It will run in download mode if I keep the battery out, but as soon as I put a battery in with it plugged into a USB, it resets the phone. I was able to flash XXUGMK6_BTU onto the phone, but upon resetting the phone with the battery in it, it will not start. This is with both ZeroLemon batteries, and the stock Samsung battery.
Is there a way to fix this?
Phone: Samsung SGS III International i9300
ROM: OmniROM Official NIGHTLY
Kernel: 3.0.101 Boeffla-Kernel-2.4-beta2-Omnirom-NG-i9300-20140805@andip71 #1
O/C Level: 1600 MHz
Media Card: Samsung 64GB Class 10 MicroSDHC
Battery: 7000 mAh ZeroLemon