This is almost always a hardware malfunction. You need to try using different hardware (different card reader or different USB cable/USB port) to transfer the photos from the camera to computer. Also try a different computer to rule out that the hard disk is the problem.
Thanks for the input. I have had this problem while importing directly from the camera via a cable, or using a card reader, and into two different USB ports. It does not occur on my desktop iMac. Today, I copied a series of RAW files shot with a Leica D-Lux Typ 109 into a folder on my hard drive, and tried importing from there. Same purple bands. Then I removed the files from the Lightroom library, although not the folder, and re-imported them. No banding at all. It seems to me that this would suggest a software glitch, but who knows?
Hard disk is the problem.
Lightroom only displays whatever information the hard disk sends to it.
Or possibly your computer's memory is bad.