Hello Artiface,
I had the issue myself, maybe your situation is similar;
When my hot-end heated up above a certain temp, the soldering joint of the heater wire would let go (cold joint)
The wire would bend over the heat and no contact would be made.
After cooling down, the heater worked again, up until a certain treshold. It took me ages to find out!
Alternatively, check if the max heat setting in the firmware isn't around 230C