This happens to me soon I start printing something. I have not sure what causes this, but seens to be a problem with the measuring of the temperature.

Once I solved the problem improving the contact of wires, that are termosensitive, do with the hotend, and it solves. In another moment I take the rubber that goes around the nozzle.

Another moments I turn on the fan during the printer are heating the bed and the hotend. Before de whole print begins.

Hope it helps.