My iPhone 3Gs randomly shuts down ever since I did the 4.0 upgrade. I downgraded to 3.1.3 but it still happens. I can't pinpoint when or why. I have read that this is a common issue. Not only does it shut down but it won't start up again unless it is plugged into a power supply aaaarrrggghhh!!!!! Has anyone figured out a fix for this yet - it is driving me insane!! Thanks!!