Burning is not successful anymore :(

hi,
hope someone can help!!!
I have been burning avi files through nero without any problem…up until a few days ago!!! now if the burning process doesnt fail the end result is a film with large blocky pixels that appears randomly through out the movie making it impossible to watch!!! im close to pulling all my hair out trying to sort this problem and im wasting hundreds of dvds…please can someone guide me to where im going wrong :frowning: oh and also its not just nero it happens with but also roxio, and powerproducer!!!)