i have been having problems with the last couple of Mega2560's that I bought, are you using a ethersheild with them? seems that when i have the ethershield on I can't have a external power supply connected. If I do, and I have any device using 5v the mega overheats and resets after about 2 minutes and then just continually resets. If it's plugged in with a usb cord it works fine.