Audio/video sync problems are almost always caused during the conversion process. One common cause is using mp3 variable bitrate audio in the avi.
Some conversion programs can deal with this better than others. You say you tried AVStoDVD? That is normally a decent conversion program. A couple others to try are FAVC and DVDFlick, both of which will produce a finished dvd ready to burn. Use the HC encoder in FAVC.
ConvertXtoDVD is a commercial program that is popular around this forum, and should give a good result if you don’t mind paying for a conversion program.