Thank you GreatGrizzly,
That is a new value to me, as I am using an earlier version of Slic3r.
I believe 12 centimetres per second feed rate is probably faster than the extruder can melt filament on a continuous basis. but this is the Maximum setting not the normal running setting.
and the rate at which you can use the filament is a factor of the nozzle diameter filament temperature and a host of other factors.
if my maths is correct a feed rate of 120mm/s would try and move 288cubic mm/s into the extruder ,
I believe the E3dV6 extruder has a melt volume of 11.3 Cubic millimetres/s.
I believe it is likely that you are trying to over drive the extruder and it is responding with missed steps.
I don't expect your extruder is going bad, I suspect that other settings in your profile may be over ambitious.
I try to make suggestions that I believe to be well founded and safe,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions,
what you do, is YOUR responsibility.