When this topic/forum post first came about, I posted in here with the same issue. I have a Panasonic AF-100 and the AVCHD clips when used natively in Premiere CS6, would have the same problems you're encountering where playback would jump around the clip, audio would come off sync, and playback in general is just very unsmooth. These 2 issues (spanned clips) have to be tied together. Overall Premiere currently does not play well with AVCHD, meanwhile Premiere 5.5 seems to work fine.
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