welcome to the club. You just trapped in the most critical weakness of the K8400. You may search the forum for a lot of other guys sharing a similar fate like you. The key factor is the thermal link between the heater block and the thermistor. A good thermal contact is essential for a proper thermal control.
I had good luck and got only some clogged nozzles, which I damaged by the attempt to make them running again.
Because of the several problems and misunderstandings as a beginner I did a comprehension of my personal experience to share with any interested K8400 user: