You'll be pulling too much current I think - with 12V in the small on-board regulator
has to drop 7V for all that current and is overheating. These devices normally cutout
but they also limit max current pulses so that when things are marginal the 5V will be
dipping when some device pulls extra current. The LCD backlight and SDcard
together will be demanding significant current.
Suggest using a USB phone charger to power the board - that way you get 5V
directly without needing a computer or a DC-DC converter.