The set up is running from a 12v 2.5a Supply, into Vin.
OK, so how much current can the on-board regulator supply? I'm not sure myself, so it's worth looking up. Remember that the regulator will be dropping 7V, so it'll be dissipating quite a bit of heat.
In my experience, bizarre hardware behaviour can be caused by marginal power supplies. I certainly had the GSM Playground crash unexpectedly when I didn't have it connected to a LiPo battery.