No one here can give any confirmation to you as this is a problem between your burner, its firmware and the media you're using. The burner is probably using a generic write strategy that may have had worked before but isn't working now, and you do know that each DVD+R DL disc isn't exactly the same as the other, right? That's why proper support for the media is a must, so that the burner knows what to do when the writing quality isn't what it should be.
The Optiarc AD-7240S is a great burner and I'm also getting one in the very near future