This behavior indicates file corruption. There are a few likely culprits. First is a memory card that is going bad. Did you check the files in Lightroom or Photoshop on the other machine? That would exclude that if they don't show the problem. Second is a bad card reader or bad USB cable. However the fact that they start corrupting after a few days even after yu already did some develop changes indicates that is probably not the case. Last is hard disk failure. Many of your symptoms sound like your hard disk is going bad. I would check it out ASAP.
Thanks a lot for the answer, the first technical answer that I received until today.
I follow your points: the memory cards are ok, tested all; I checked the same file from SD cards in other machine and they were ok. The card reader (installed in my PC) is ok, there isn't any cable between card reader and PC. Moreover, I've never seen corrupted photos from the D750 lcd, so something damage them after shooting.
Then, after this consideration, I suppose that the problem should be really my hard disk!
Thanks again for your collaboration!