Audio out of sync

Can anyone tell me why some of the movies that I have downloaded play on my mac fine but when I burn to dvd the audio is out of sync with the video and is there anything that I can do about it. I am using iDVD to burn. I also have problems with iDVD getting to one minute remaining in the burn and going no further. I have left it over night at this point and in the morning it is still at one minute.