zuluclayman

If it is a longish movie maybe your computer specs aren't up to the job - what RAM, processor and video card are you using? It sounds as if you may not have enough grunt to run Vegas efficiently for larger movies.
Post your system specs and lets see.
if this is the case you can try rendering/exporting portions of the movie at a time then put them together after they have been compressed (and are therefore smaller file size).