Federal aviation authorities identify a new problem in the Boeing 737 Max that they want remedied. The problem involves the uncommanded movement of the horizontal stabilizer, the wing that moves the plane up or down. The discovery of the problem could further push back the return to service for the jet grounded during airlines’ busy summer travel season. The plane was grounded after two deadly international crashes in March and October. It was unclear whether the new fix will have to go through rigorous, time-consuming testing in a way that will further push it back.”]