Hmmm?
What type of file is it? If it’s an uncommpressed HD file like an .avi that’s most likely the problem. HD files in most cases should usually be an Mpeg 2.
Another thing that sometimes works is to make a small adjustment to the clip like anti flicker (right click on the clip and select field optionsm then anti flicker). Then you will have to render the clip. After it has rendered see if it plays smoothly.
If that works, you can make any change you want that would make you render the clip (Doesn’t have to be anti flicker). Rendering seems to do something to make the clip play smoother.
Let me know.