Drone, mavic pro, create a polygon mission, problem: the drone goes one way at a fixed speed of 9-10 m / s and back 1-4 m / s
and so every line.
Phantom 3 works stably on the same smartphone.
The behavior you are describing has not been reported to us so far. As a first step, I would suggest you go through all the basic checks and troubleshooting steps as it will solve most of the issues that can be encountered if properly followed.