Do your renders in FCPs Timeline suddenly seem to un-render themselves? A friend called me a few moments ago in a panic saying that FCP was freaking out and un-rendering, seeming at random.
To make a long story short: he didnt realize that a previous projects test Render Files folder was still being used by FCP...while it was in the Trash.
Solution: Drag it out of the Trash or tell FCP to use another folder (stay organized, though).