It sounds like the raw data is corrupted in your files. This could be anything from your camera, the card, the reader/USB-cable, the hard-disk or RAM, but usually the card or RAM.
A way to narrow down where the issue is would be to transfer the photos, again, using a different method, like a reader via the OS rather than the camera using LR, and see if the corruption is still there when you import them a second time—either turn off the duplicate checking or use a new catalog for the experiment.
If the same files are corrupted in the same way then it is likely that the data on the card is already bad. If different photos are corrupted or the same photos but in different places, then the problem is likely on the computer, and RAM is the usual suspect so a memory test might be in order.
I am having the same issue.I import RAW/NEF by putting the memory card directly into the card slot in my iMack.
Photos are distortet right after import, or any time after. I agree; this could be a hardware issue. It seems to appear only to my RAW photos and the photos on the camera memory look fine.
What's bugging me is why this happends, becouse it shows up occationally on both edited an non-edited photos
Most likely, the raw data in your files is corrupted, mostly missing in the above example. The reason it looks ok on the camera LCD is that camera is showing the small embedded JPG at the beginning of the NEF which is more likely to be intact than the large area of raw data at the end of the files. You need to try another card and see if things work, and if so, throw away the memory card that has the corrupted images. Depending on how important the images are, there are ways to extract the embedded preview JPG, what is visible in the Import grid, so you have something to work with. The images will end up w/o any EXIF information so you also would need to transfer that from the NEFs to the extracted JPGs. The way I remember how to extract the embedded preview is with EXIFtoolGUI in conjunction with the latest EXFtool:
Edit: I accidentally included the wrong link, initially, which fixes the corruption from using an older version of Nikon products on a new camera's raw files. This is not what your issue is. The new link, above, includes ExifTool and ExifToolGUI.
Your memory card may be bad, or the files could be corrupted during the transfer from card to computer. Try formatting the card on you camera first, and then shoot some test images. If the problem is still there, get a new memory card. If you still have problems after that you need to get a new card reader to transfer the images.