It could be a corrupted render file. When I run into something like this, I take my files offline (unmount the media drives) to narrow things down. You could try just taking the render files offline first. Quit FCP, then add an “X” or something into the Render Files folder name, then re-launch FCP. See how it plays. If this solves it, I’d just trash those old render files and re-render.
If this doesn’t solve it, try taking all your media drives offline, or move QT files out in groups so you can narrow it down. You’ll have to close FCP everytime you do this, mind you.
Billy.