I have the same problem, and it seems to happen only when I go full screen on AVCHD clips. I have a GH1 but that is not the problem, as this happens also with Canon HF s100 clips.
I personally think this happened after Snow leopard. Have you gotten any other feedback..
If you take the same clip, reveal it in finder and just “preview” it by hitting the space bar, does it do the same strange thing? Mine does not. Only inside of FCP….
interested in knowing if anybody else have this problem. It is irritating..
THomas