Had a similar issue with my ceramic straightener last summer. It would turn off after ten minutes or so and no warning. Annoying as hell. Cleaned the plates thoroughly, thinking maybe residue was causing it to overheat and trigger the auto-shutoff.
That didn't work, so I tested the outlet with another appliance. Turned out the outlet was faulty, not providing steady power. Switched to a different one and problem solved. Simple fix. If that's not it for you, maybe the heating element is wearing out from all that use on thick hair.
