First, Thank you for your answers.
Unfortunately I tried again and again today and I can't properly flash my winbond chip. I even tried to nice -19 flash_rom, remove the print's but it doesn't help.
I dumped several time the content burned, and there are each time several errors, not twice the same, randomly placed across the chip.
The wrong bytes are often burned as FF, but this is not always the case.
What else could I try ? Where should I dig into ?
Thx Damien