That was my next suggestion, to make sure you have the latest version.
HD files can play back pretty sluggish on some of my editing stations in media player or quicktime, choppy or out of sync. But in my editing software they are fine, since the software plays back low resolution proxies for editing.
First, I'd make sure there were no unnecessary background programs open and active on your computer to use your full processing and ram power.
If that does not help, you could try compressing the video from 1080 to 720 or smaller to have a file that plays back easier just to ensure that the audio and video are synced.
These would be my next steps just to make sure the video is ok to begin with! Hope this can help. Video can be a tricky bastard sometimes.