Final Cut Express seems by default (or maybe I did it) set the default Render frame rate to 50% to speed up the NLE previewing process.
These render files however are the same ones that are used to put together the ‘finished product’ if you don’t use one of the Quicktime Conversion options.
If your output (to DVD or whatever) is choppy and you think the frame rate is possibly messed up the easy way to check is to pause your Quicktime movie, then use the left and right keys to move through frames. If each pair of frames is the same then you have not got a full output. To resolve it, go to your User Preferences screen, click Render Control and select 100% from the Frame Rate drop down.
I did fix this problem several months ago, but seeing as I am waiting for something to render I thought it was an opportune moment to write it down.
Update: pssst don’t forget to try deinterlacing if your video looks rubbish ;)