Sync problems with sound from FCP --> Cubase --> FCP

HI,
I'm experiencing something weird. I've been mixing a trailer for a company the whole day in Cubase 6 and I exported the whole project as an OMF from Final Cut Pro 7 and imported it to Cubase. After finishing the mix I made an "Audio Mixdown" and imported the file into FCP. Things were out of sync the further you got into the trailer and started already out of sync quite early.At first I thought it was the problem described in another forum, that if FCP has different sample rates on different files, it will not convert the files in the OMF and therefore create out of sync material. But then I converted the samplerate on the files that were 44.1 kHZ in FCP, exported to OMF and the same problem appeared - though a little less out of sync in the beginning.Then I manually exported every single track out of FCP by choosing "Export - Audio as AIFF" and imported to Cubase. Mixed and exported with "Audio Mixdown" and imported the stereo file into FCP. Still out of sync at the end of the movie.So then I thought... maybe it's a framerate problem? I checked what format the video is in FCP - it says HDV 1080i50 25 fps. I checked in Cubase to see, that I work in 25 fps. Yes I do!!So now I'm lost!! How can my stuff be out of sync while working in the same framerate and samplerate!!!Any ideas would be VERY helpful
Thanks

I did not find the right solution from the internet.
References:
www.steinberg.net/forums/viewtopic.phpt=6263
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