When you upgrade or install a new version of FCP, all its settings are reset. So, a lot of your sequence settings, playback settings, RT settings are reset. You may have set them in FCP 6 in such a way that renders and playback of effects laden shots were affected in a certain way.
Which is why, when you went back to FCP 6 everything went back to normal. Also, you opened a project created in a previous version, in FCP 7. That too affects things.
And, for a while capture card makers will have drivers that aren’t up to speed in the effects playback department. So bear with that. It might help to inform the card manufacturer with a specific issue. They often may not reply you but they will certainly try it out and even quietly correct it in the next release. Has happened to me.
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Neil Sadwelkar
neilsadwelkar.blogspot.com
twitter: fcpguru
FCP Editor, Edit systems consultant
Mumbai India