In your FCP prefrences there is an item called Auto-Render. I think the default is 45 minutes. In other words, if you step away from your workstation and nothing happens for 45 minutes you will see FCP start to render your sequence (if there are items to render). So, I just set my Auto-Render to 1 minute and wait to see if there are any items it needs to render before I output to tape.
To be honest, I’m not clear myself what it renders (especially when there are no red or green lines for audio or video). However, what I do know is that I’ve experienced the exact same thing that you are struggling with and when I started letting Auto-Render do its thing the problem went away. I started doing this with FCP 4.5 so it’s not something exclusive to FCP 5. I hope this helps.