iPhone 4S charging problem

Hello,

My iPhone 4S has been having problems recharging. The problem is that once I want to let my iPhone charge from a wall charger, the phone always has like a battle to be either charging or not (the lock screen always goes from the battery charging to my wallpaper and then back and back). I have changed the USB cable and used other wall outlets, but the problem still exists. One weird thing is that it charges perfectly fine if I hold my iPhone upside down with the dock connector on top, while held upwards, it won't charge.

Please help!