Have you tried "wiggling" the charger while it's plugged in after it quits.sounds like a loose connection. With Micro USB, the part that makes the electrical contact inside the phone is like a small "blade" and can get bent or even broken semi easily..... if that happens it might not make good contact. So when you first plug it in, it makes that connection, but when you set it down, it looses that connection, and it takes the phone a few seconds before it displays that it lost that power source.
