What's the power source?
5V is coming from the 5V header pin?
The regulator is limited to 800mA.
It will be less if it has to regulate higher voltages down.
I don't use the barrel jack when I can - instead I use a 5V wallwart rated for 1 or 2 amps and power things that way. With a Mega, adding a diode from 5V (anode) to Vin (cathode) will prevent backdriving the regulator and damaging it.