Audio goes out of sync with Soundtrack Pro

Hello - I'm using FCP2 on a 2x4 Mac Pro and sending the audio to STP to reduce background noise, however each time I go back to FCP the audio is out of sync. The start point and clip length stay correct, though the syncing issue gets increasingly worse towards the end of the clip.

I've tried sending to STP as Multitrack project and Audio File Project but get same issue with both.

The footage originated from a Panasonic HVX200 @ 720p, 16bit 48kHz

Why is STP doing this? And more importantly, how do I fix it???

Much appreciated.
David.