The on board voltage regulator can't handle that much load. Use the USB connector instead, plug it into a phone charger wall adapter. Maybe the adapter you're using is faulty or can't supply enough current.
If that doesn't work, maybe there is a real hardware problem. Can you post some close up images of the shield installation? Check the bottom of the shield for any cut off component leads that are too long and so contact things on the Arduino like the shield around the USB jack.
Is there any additional hardware you didn't mention?