Maybe it stops periodically to prevent over heating of the circuit inside.
I don't have an induction cooker yet but some of the steel utensils show rust. I think its because of the highly corrosive type of dish washing liquid we get now combined with the metal or that green nylon corrosive utensil cleaning pad. I can see utensils having some black powder on them even after washing them with clean water twice. The dish washing liquid have some buffing agent in them which probably corrodes steel on atomic level. Rub a finger on a steel plate and you'll know what I'm talking of.