From the description, it appears that this is a problem known as a judder. The judder results from irregularities in the design of the friction material. This causes changes in the disc material, while increasing the coefficient of friction in the central part of the disc. When various braking-related processes, such as pressure, speed or temperature, coincide, the contact between the pad and the disc results vibrations. The best option is to replace the pads and discs.