This looks like corruption caused by faulty hardware - could be a faulty USB cable, card reader, or card.
If you are connecting the camera directly to the computer, try using a card reader instead.
Raw files contain a jpg preview, which is probably what you're seeing briefly when you click Reset.
This is a hardware malfunction somewhere, causing the files to be corrupted. It could be the camera card, card reader or USB port/USB Cable, or hard disk, or memory.
You need to try importing using different hardware, and if possible using an entirely different computer.
The fact that they look fine for a second or two is because Lightroom is showing you the JPG preview embedded in the RAW file, which apparently has not been corrupted by the hardware problem.
Some of this is a repeat of what others have said...
First off, the raw data in the photos is missing or otherwise corrupted. LR did not cause this corruption, but the initial view is of the embedded JPG preview the camera put there and it's smaller and probably ok. The corrupted rendering is when LR actually processes the raw data, itself.
Usually it is the card that is bad. But if it is the cable or the reader or loose connection then sometimes transferring the files, again, yields different results. I'd suggest doing that but using drag-and-drop in Explorer/Finder instead of using LR and switching between using the camera to a card reader or using the camera instead of a card reader. It can also be the hard drive or memory in the computer, but that's less often the problem.
If you've already deleted the photos and shot more then I'd suggest discontinuing use of that card immediately and see if the situation happens, again, with a new card. If it does then something else besides the card was at fault. If your camera has a way to write photos to two cards then switch that on, the mode that creates duplicates on each card, not the mode that saves a JPG on one and the raw on another.