Unexpected shut down via low battery not really low battery iPhone 3G

Running version 3.1.3, take a picture with the camera, open safari, open a picture in a email, etc. sometimes the phone just shuts down. Battery will be 90%-100%, however immediately shows a low battery and will turn off. Hook up to either a wall charger or via computer USB and the battery returns to 90%-100%. Hard resets, restore, etc. all have no effect. Sent the phone in for a Battery replacement to apple, they returned the phone with out replacing the battery and indicated that the battery was fine and they could not duplicate the problem. If I use the phone only the battery lasts for days, any ideas out there?
Thanks
Greg
Powerbook 17", Imac 20" Mac OS X (10.6.4) Live type, Photoshop 7, Illustrator 10