Best advice, keep all other devices power supplies separated from the arduino.
I don't know this coin-validator , but i presume it uses pulse signals for the accepted coins instead of any protocol communications.
Use separate powersupply for power and use opto-couplers between signal wires and arduino.
I think the arduino is ok with 12V but NOT with 12.8V, so better NEVER use higher power for arduino then lets say 9V. And never supply any voltage to any hooked up device from the arduino.
I think you can even damage your board if you start driving shhiitloads of LED's without extra external powersupply.
I found out i needed a desktop powersupply with +12, -12, +5, -5 and 0/ground to do all these experiments