You have something causing a hot spot. The timing makes no difference since the plugs aren't firing with the ignition off. acwhite is on the right track by suspecting a carbon build up. Setting you idle down can help sometimes but to FIX it you need to eliminate the hot spot that's acting like a spark plug when you turn the key off.

Save a life, adopt a senior shelter pet.
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Name your dog Naked so you can walk Naked in the park.