I had this problem. I also noticed if I let the iphone run out of power, the screen would flicker for hours after I charged it. Weeks later the battery finally gave in, that is, it would not hold charge over night. So was useless as an alarm clock (really important for me). As I could live with the sound and screen issue, I decided to just replCe the battery. All three problems immediatly went away. I guess the battery had lost its ability to provide enough current to power everything at once.