The difference between the two is that the XBee will automatically retry the transmission. This means a longer transmission time if it can't get the message out. This will use a little more power, but it doesn't sound like enough to kill things. I'm not familiar with the shield, could it have something floating around the reset pin? Maybe pull it high as an experiment or something.
I had an arduino close to a permanent magnet motor (a big one) and the reset pin had a wire running to a switch on the side of the enclosure. The arduino would reset sometimes because the wire picked up enough noise from the motor to actually reset the board. Totally random event and it was easy to fix by filtering the stupid wire. Took a while to figure out though since my presence in the noise field would cause it to stop having a problem. Every time I sat and watched the thing to see what was going on, it quit failing.