you're using absolute junk media...i hate to say it so bluntly, but few people have gotten decent burns out of ritek DL media (or any ritek media these days for that matter). if you do a search for "ritek DL" on the forum you'll see all of the problems that this media has caused

try verbatim DL discs (MKM 001) and then update us with how that works out.

also i'm not familiar with your drive, but make sure your firmware is up to date as well.