copied from elsewhere on the interwebz
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Method 1: (thanks to sonicbuffalo for posting this method)
*DOES NOT DELETE YOUR DATA
1) Once you are in ClockWorkMod Recovery, scroll down to "Wipe cache partition".
2) Select "Wipe cache partition" using the home key or power button.
3) Now, scroll down to "Yes - wipe cache".
4) Press the home key or power button to perform the wipe.
5) When done, restart your phone.
6) Your ThunderBolt will boot into the Android operating system and will say that the UPDATE FAILED.
7) Choose Cancel to stop the update.
8) Restart your phone one last time and you'll see that the "Powering Off" loop is gone.
Method 2: This will DELETE your data (do this if method one does not work)
1) Once you are in ClockworkMod Recovery, select "wipe data/factory reset" using volume keys.
2) Press home key or power button to select.
3) Answer Yes, by scrolling with vol keys and home or power key to select.
4) When phone reboots, the powering off loop should be gone.
Method 3: This only works if you have a Nandroid Backup from prior to accepting the update.
1) Once in clockwork select "backup and restore" using vol keys.
2) Select using the home key or power button
3) From next menu select "restore"
4) Choose your backup from list and select.
5) Answer yes to restore backup
6) When phone reboots the "powering off" loop is gone.
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