problem with DSLR video


I try to shoot action sequences with a DSLR (Nikon D7100) at 60 fps 1080p and a flycam to stabilize.
When I view the video, I don't see any problem but when I stop on a frame on hitfilm, I see strange artifacts (here is a screenshot where the arm is distorded).
Any idea on what I can do (camera settings, post correction,...) to avoid these artifacts?

I also have another problem :
Sometimes, some frames of my clips freeze on hitfilm (even if I do frame per frame, and at export), and the only way to correct that is to reload the clip. Any idea on how to correct that?