Hi everyone. I have built a Kossel XL printer running with E3D Lite6 and currently I have been succesfully printing for a few weeks with PLA. Today a weird problem appeared that the heat of the heater block fluctuates a little bit while printing and it acts like a sine curve (please see the attached images). It is +-3 degrees right now, till yesterday I did not have any issues like this, the temperature was stable while printing. Do you think it might be a thermistor related problem or is it something bigger?

Thanks a lot!

You are using PID-control for the hotend?
It looks, like a parameter has changed, so the PID coefficients are no longer working.
The changed parameter could be raised wire/clamping resistance or thermal contact issues of the heater or thermistor in the hotend.
-Olaf