Q: Iphone 4 blue screen on startup but ok afterwards

Hi,

When I turn on my Iphone 4, model A1332, it often starts as if the vertical sync has failed. I can see that I am on the start screen in that it is the right general colour but the screen is just vertical lines. If I do an upward swipe the controls screen comes up as normal, screen brightness, volume control etc. When I close that screen I am back to normal and can see the start up screen with the time and "swipe to unlock". As this is happening more and more regularly, any idea what the csue of this is?