I can think of no reason this would happen, Mark -- short of some weird glitch (which is why I suggested you try it a second time). But if your movie is going buggy at the exact same point as the first time you created this output, then something much deeper is at work.
Try one more thing: Go to Share/Computer/MOV and output it as a Quicktime movie. Check to see if you get the same results.
If not, then something is just buggy about that output preset and we can work around it.
If so, then something is corrupted in your project, and you may not to try making a new project.
