Audio losing sync after files are imported to FCP?

Hi Guys. Bit of an odd one and i've been digging around but haven't found anything yet.

I am having an issue with footage in FCP losing audio sync to the extent that even though the timeline clips appear to be the same length, the audio starts slightly earlier and is cut of at the end of the clip. It 's almost as thought the audio and video timelines are running at different frame rates but the problem starts earlier than that.

I am cutting a showreel together and it's a bunch of 30sec tvc's. All of them play fine off the desktop no sync issues at all but once imported to FCP the audio is all over the place even before i put into the sequence. At first i thought it may be a processing thing so i opened up couple of music clips that i had cut that were a little more sound and graphics intensive. They played out fine. The Media is coming of a 12TB RAID5 with 9TB usable storage. All the disks are in good health. I'm running 7200rpm seagate barracudas and never had problems before today.

I tried opening up a new project and starting from new but still no joy. I was running audio through external speakers which would account for a delay but not to this extent. and i also tried just using the built in speakers to be sure.

The video is compressed to ProRes 422(HQ) 720p 48khz sac audio data rate 79mbits

Thoughts???

Thanks

J