You could try importing one clip to your mac and running it right off the system drive instead of an external. If it's still choppy you'll know there's something else going on. Having said that, the newest Mark IIIs have a new codec, so that might be part of the problem with old software like 5.5.
Also, an old workaround might help. Try renaming the clip as an .mpg rather than a .mov before import.
