Over the past few months my 3GS will shut off and not turn on as if the battery were dead intermittently at around 65% or 35% battery life. Is my battery just on its last leg? Should I bite the bullet and upgrade to 4S? It is very frustrating writing an email or something only to have the phone cease at 65% and not be anywhere near a charger. Has anyone else encountered this issue? I searched a bit on the forums and didn't find any info.