Arduino Rebooting

Other Hardware Motors, Mechanics, Power and CNC
2

Looks like "digital out" and "digital in" are reverse labelled in your diagram.
My guess is RFI or voltage transients falsely turning on the thyristor. I agree that some sort of filter (snubber circuit) would be needed.

http://www.fairchildsemi.com/an/AN/AN-3008.pdf