Is playback set to “full” or “dynamic”? (Or at least I think that’s the correct wording — might be “full” or “sequence settings”.)
One way to check that FCP is playing full resolution is to begin the process of editing or printing to tape, On the ready screen, if the setting aren’t right, you’ll get a message saying the playback might not be a full quality and tell you what to change.
I’ve noticed that even on rendered disolves, if the setting aren’t right, I’ll get a noticeable drop in quality during the dissolve – including the picture appearing to shift.