At first I thought it was a bug in my code, but after testing several different Mega2560 boards with the same code I found that the voltage regulator in that block was defective. With a 5V supply with the USB off, everything works fine, but when using the 12V power connection on the plug, the voltage regulator overheats and the board starts to work erratically, randomly shutting down and restarting. - EDIT - After contacting the manufacturer and offering to replace the circuit board, it turns out I had a bad unit and the replacement works as expected. Have changed my review to reflect this: sometimes things go through quality control but how the manufacturer handles the situation is just as important as the product itself and they took care of it quickly.
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