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That phone looks like it has caused many people, problems. Bugs are many.
I see that you are doing the usual and knocking items off the list.
A few posts in this linked thread have actual information...
Based on that linked thread, what you need to look for is:
What app or system process shows high usage(relates to the battery) in the monitor? Is it the OS? Delete useless Samsung apps that won't stay off.
What OS version are you running? 2.3.4 seems to be ok. 2.3.3 light killing battery. 2.3.5 and 2.3.6 shows battery killing time.
Some of the posts suggest its a wifi/cell radio issue, where it does not shut down fully when not in use.
And a whole lot of other things suggested that worked for others:
USB port has a flaw. Clean it if has grime so charger plug gets full contact
Battery not calibrated to the OS and is not charging properly. So if the phone is off, is it seeing the battery actual charge state? Results in strange charging times.
SD card causes problem. http://forum.xda-developers.com/showthread.php?t=2470091
Phone keeps calling to the tower and sending tiny data packets back and forth. Radio on all the time