I'm not sure I'm understanding your problem correctly, but if the phone is showing 88% charge when you know it to be about to die, try this. Let the phone run completely dead, then let it sit that way overnight. In the morning, plug it into the charger, and leave it on the charger for an hour or two after it hits 100%.
Many lithium/ion batteries have a computer that controls/monitors the battery, and it can get out of whack to the actual battery condition. I've noticed this is even more common when the device typically sits plugged in at full charge for long periods of time. Running the battery to full zero then to complete charge normally gets the computer back on track.