Purple lines in the photo almost always indicate that some piece of hardware has caused some file corruption. It could be any piece of hardware in the transmission of the file from the camera caard to the computer. One obvious thing you can try is to move the photos to the computer using a different card reader/different USB cable. If possible, you could try copying the photos to an entirely different computer and then importing them into Lightroom. Run diagnostic checks on your hard disk and computer memory.
Other software may show the file has no cuorruption, but it is usually displaying the JPG preview that is embedded in the RAW file, and this JPG preview very often is fine when the RAW image itself is corrupted. So the fact that other sofrtware doesn't show the problem is irrelevant.
Thank you ever so much. I will immediately try what you suggest. You may well be right, I have had problems with card readers in the past.