How can I fix my De Dietrich Piano induction hob ?

Hi Glyn, is it at all possible that the display is showing an error code during that time or you've not realised that what comes up on the display is an error code? Let us know what this code is, if there is one coming up.
With no error code combined with what you are describing, my guess that this a main control board fault and that would need changing. You could at least try isolating the main power for a minute and then powering back up to see if that clears a glitch, I doubt this will help, but can't harm trying.

Rupert
May 2020