Going to need more information than that. But if I had to guess I'd say you are burning movies, originally you were using movies that were in the correct format to begin with and now you are using something else which is being converted to the correct format before burning.
Either that or you are continually buffer underruning or burning with the drive in PIO mode rather than DMA. Google it up if you don't know what I'm talking about.
Also, it would serve you well to read the link in my signature.