Hi,
My problem seems to be solved.
For a reason i don't realy understand, the mouthpiece thermostat was not ordering the right temperature. For exemple, when i was adjusting it to 300 °C, i think the heater was in fact trying to heat until maybe 500 °C. The result of this was a mouthpiece and the metal far far too hot.
So, I had to change the setting of the themostat in order to, at the wright temperature, get the indicator on the scale. This way, i'm able to read : when the indicator is here, it is the right temperature.
The slugs are now very good, and the temperatures of both the metalpot and the mouthpiece are stable.
Thanks a lot for your indications!