but, so far it has been a riddle. I have another, equal pen, with the same nib, and it doesn't happen, so:
1. it is not the nib design (but could be a nib defect); 2. I have tried different inks (but am willing to keep trying with new ones). 3. The pen has that problem since new (so, the clogging inside the convereter may not be the culprit); 4. It happens with the piston converter, and with cartridges. The pen never stop sthe ink flow while writing. I did clean the space between the tines yesterday with a razor, and did find dry ink, so, somehow, the ink is drying between the feeder and the nib pretty quickly. Anyway, thanks for all the advice. I'll certainly try again all your suggested solutions.