Upon more analysis, I have discovered that Movie Studio actually is adding frames causing this motion to happen.
The video as I view it in PMB is about 500 frames long. Each frame is unique.
When I import the same video into Vegas, I see for example that everything looks fine up to frame 63. And then frame 64 was inserted and is identical to frame 62. Frame 65 looks like it carries on from frame 63.
So when the movie plays at normal speed – say of someone running – the runner is running forward then at frame 64 steps back.
What could be causeing Vegas to add the frame that doesnt exist when I view it in PMB?