If they are dl`s from the net then suspect they would be avis. Use DVDSanta (free prog google it) to convert to dvd format (VIDEO_TS & AUDIO_TS files) then use your normal burning prog (I use stomp recordnowmax 4.5) to burn it.Never had a prob.
Even more info when using dvdsanta you can select either widescreen or full screen to convert to - use whatever the avi file is ie, load up the avi in windows player and see if it plays in widescreen or not and select accordingly in dvdsanta. Also you have an option for PAL or NTSC, right click on the avi file and go to details and it should give you the fps.25fps is PAL and 23 or 29fps is ntsc. If you get this wrong then the audio will be out of sync with video.
DVDSanta is easy to use and takes about 45min for a normal movie.