Why are quicktime files from my camera distorted?

I shot some video on my Nikon d5200 on two separate days and now playing the videos back on my computer, they look terrible during motion. Like when cars go by, its fine until they get close and then its a bunch of bars. Same thing with trees, when they get close on the footage, they don't look like trees anymore, its like a bunch of bars and pixels. Why is this? I converted to mp4 and it does the same thing but with even more bars across the whole image… Please help, I'm so lost.

Added (1). The video files play back fine on my camera screen and on google drive…

Your computer is not powerful enough to play your videos. You may need more memory, a better graphics card, and/or a more powerful processor. If your computer is more than about 4 years old, or is a very inexpensive model, it simply doesn't have the power needed for reliable video playback.

Al AlCapone states, may be a computer issue if your movie uses variable bit rate encoding. If you're using Windows, use Media Player Classic Home Cinema or VLC to play your videos. Both of these movie players are free and work well. The Windows movie player (any Microsoft product) is garbage.