Maybe something's up with the fixture. All of our ceiling fixtures have a thermal sensor and shut themselves off if they get too hot. If one of those malfunctioned, it could muck up the power.

And, never underestimate the ability of an electrician to do something stupid. My father had some flakey ceiling lights in their new condo (built in the early 80's, renovated at least once since then). After much debugging, it turns out that some earlier electrician needed to extend a wire so just taped the splices without so much as twisting them, much less using wire nuts. Redoing the wiring fixed the problem.

(And the breaker box had evidence of a history of sparking, based on the charred bits within.)