QT export audio out of sync

Hi - I'm having problems exporting. When I export HD QTs the audio is half a second ahead of the video. Playback in FCP is ok, but not great ... it often stutters and is out of sync but after pausing it, it then plays well. The Quicktime exports play better in smaller frame sizes. I feel the computer is chugging and struggling somewhat.

I've tried the following to no avail:

Quit and Restarted FCP & Computer

Trashed my Preferences

Checked 'recompress all frames' on export options

Ensured I have 15% free on both HDs (edit files are on external HD)

Started new sequences within the project and pasted in the edits

FCP Version 7.0.3

Mac OS 10.7.5

2.66 Inctel Core 2 Duo iMac

The footage is XDCAM EX from a Sony EX3. I've attached photos of my setup & sequence settings.

I've spent a couple of hours re-exporting and looking online for help but nothing seems to be working. Its frustrating as its a workflow I've done for a couple of years ... if anyone out there could help it would be amazing!

Many thanks

Oliver