I've had two new pens which wrote unevenly:
Sailor King Of Pen
Omas Milord (the last version with converter)
To my surprise, a wash with water containing only 2 drops of liquid soap in a cup of water fixed the problem. I suspect that the fix was not cleaning the pen, but rather adding a trace of soap to the ink. Because I did not rinse them with pure water after the wash, so there was a trace of residual soap in the nib and converter.
If either pen returns to uneven writing after several refills, I'll first test only adding a trace of soap to the ink before re-washing.
Edited by Precise, 13 January 2018 - 20:32.