sob, you need to reset battery stats. I've seen 4 different ways to do this, and while I'm sure all of them work in their own rights, I'll give you the directions to what I do time and time again.
Charge phone to 100%
Unplug charger, plug back in, charge to 100%
Turn off phone, charge to 100%
Unplug charger, charge to 100% (with phone off)
Turn phone on, using ROM Manager, boot into ClockworkMod Recovery.
Open ROM Manager app
Flash ClockworkMod Recovery (for Captivate)
Reboot into Recovery
When you are in Recovery (easily recognized by GREEN text), scroll to "advance"
Scroll and select "Wipe Battery Stats"
Once you wipe battery stats, navigate your way out, and reboot the phone back to "normal". Now over the next day, monitor the battery drain and see if it gets any better (which it should). I would also remove any task killer you've added as well, those don't really help you.