Power Steering is locked

Dear ML 55 AMG owners,

While driving, I felt that the power steering is not moving smoothly. It became very heavy and tough. I stopped the car at first check station, checked the steering oil, it was ok.
I started the car again, it went fine..... !!The problem disappeared.

It this a sympton of a future problem ? Have you experienced the same,
Is there any advice..

Thanks

That is unusual. I believe there is a speed sensitive sensor on the steering rack. If that failed you would notice the difference at low speeds. But it would not feel like it is going to lock up.