I’m suprised no-one has mentioned the problem of Quicktime on the PC platform (it being a bit slow to decode files in a MOV container).
Premiere could be using Quicktime to decode the files which results in choppy playback. I had this issue with Nikon H.264 files in a MOV container. The workaround was to change the MOV extension to MPG which forces Premiere to try and use its own decoder for the job. Suddenly no more choppiness.
I think it might have been Todd Kopriva who explained the problem in a previous thread.