Check the avi files on the computer by playing them in a media player like Windows Media player or VLC. If you can see pixellation in the same areas of the video on the pc as you see on the dvd player, then the downloads are at fault.
Are you converting these to dvd-video or are you simply burning the files to a blank dvd as files? Many newer players will handle avi files (especially those encoded with divx or xvid) and all you have to do is burn as regular data files. If you are converting, you can sometimes introduce problems in the conversion process. So it would help to know your process and the tools you are using.
Burning speed can be controlled in any burning program. You will either have to find it yourself or tell us which one you are using.
You have found the mid code on those TDK disks. They are made by an Indian company called Moser Baer, but I don’t know much about that particular mid code. My first instinct is to tell you to cut your burning speed down to 8x. If that doesn’t work out, get some Verbatim 16x disks and burn them at 8x or 12x speed.