Thank you for your answers.
@geert_2, I've tried to print my parts an other time and I watched the printing process more closely. I observed exactly what you describe (about the "worms" thing and the brown spot), some filament is extruded ("leaks) while the printcore travels. Also I print with two materials at the same time (PET, printcore 1, for the part and PVA, printcore 2, for the support), and it worsen the leakage problem. The moment when the support part is printed (PVA) gives more time for the PET to leak from the first printcore.
I will try to slow and cool my next print, and I also use the PET filament from ICE. Are you satisfied by this filament by the way ?
@P3D, thanks for the tip, I will test that