Possibilities:
* The DVD media you are using is causing problem with your DVD writer, try using another media.
* Clean the DVD writer using Lens Cleaner
* The lens may get replaced, time to repair or replace if under warranty. If not go for LG or Samsung
Also try burning the DVDs with other softwares like Nero and try to reduce the file size.
Try burning some other DVD, if that too fails, then surely it is a problem with your lens.
I had several problems with National... I know it is widely available and cheap so obvious for anyone to go for it.but try using moser baer or frontech atleast that has 16X written on it.. lesser than 16x caused problems for me. Use InfraRecorder for burning it.