After my last post I made a complete TT set-up, to confirm what were the causes of the distortion. Had it checked by an expert in TT setup.
I found some issues. The O-rings of the suspensions towers were too stretched and one of the towers was no more suspwnding the table. Replaced all the O-rimgs. But the problem continued.
then checked the needle with a microscope. Found some dirt on the back of the needle, hard to remove. Using the Lyra SPT with care, finally arrived to remove it. It got better, but the problem was still audible.
and not only audible by me. Clearly audible by the person who made the new set up.
finnaly, checked overhand, VTA, weight (1,71 g, on the recommended values). Everything is ok.
Finnaly, put my spare cartridge back on the table, set it up and everything went into focus again. I must conclude that the problem is on the cartridge.
I will send it to reconstruction.
however, it is amazing that only on the cleaned records the problem was noticeable. It is very strange and unexplained thing. And audible by two persons, to eliminate psicológicas effects. In fact, in some cases even a deaf would detect the distortion. And only on the cleaned records.