Ah interesting, I would think i9000 bootloaders on an i897 would CAUSE issues!
I put together a little package for anyone with an i897 captivate still having shutdown issues and is still frustrated. Give this a try, I have a feeling they will disappear.
Use this Odin one click to wipe/reset your phone to the latest official rom, uckk4. This one click INCLUDES the latest bootloaders ever released for our phone, both primary and secondary. These are the bootloaders that have made mine and others shutdown issue disappear. This is completely safe to one-click no matter what is on your phone right now, just use a reliable USB cable like with any other flash. No matter what state your phone is in, if you can get into download mode, this will wipe your phone to 2.3.5 with 2.3.5 bootloaders
http://fohdeesha.com/data/other/android/i897uckk4 with latest BL.zip
After you've done that, use this odin package to install CWM recovery for the stock 2.3.5 installation you now have
http://fohdeesha.com/data/other/android/uckk4 CWM recovery.zip
Now you can get into CWM and flash whatever rom you want! ICS, jellybean, whatever. And you now have the latest 2.3.5 bootloaders and your shutdown issue should be gone. I've done this several times to several captivates now with zero issues. I hope this helps someone!