Do a test, burn one with that option enabled and see if you see any hickup/issue when your player plays over the layer break.
Remember the chapter you selected as layer break and just note when it moves from chapter x to chapter x.
Normally Necs book type +R DL, Can you see in the burn log that it didn't?
The DX-20A6Q seems to be a rebadged Lite-On. If you use the Lite-On tab in the book type function do you get errors?
I 02:23:52 Destination Media Sectors: 2.295.104
W 02:31:59 Failed to Write Sectors 2295104 - 2295135 - Reason: Logical Block Address out of Range
Looks as there are no more sectors on the disc to write on.
Sony Optiarc AD-7240S - Laufwerk - DVD+/-RW (+/-R DL) / DVD-RAM - 24x24x12x - S-ATA - intern - 13,3 cm (5,25)
this one is good for DL burning?
It's nice. You can see test burns here.
http://forum.imgburn.com/index.php?showtopic=10843
Could be an underpowered power unit. Had that issue ages ago with to many optical drives installed. Seems it uses more power when you actually use the drive than when it's idle and then it gave me a blue screen.
If you get burning issues with quality media on single layer media also, that could indicate it. Single media burns ok with this burner, it's DL media that is the problem.
It could came a firmware update that solves the problems. But it has been months since the latest one and I frankly doubt it will be here in a short time.
Your issue could also come from that your SATA connector on the mother board doesn't like optical drives. My Asus suffers from that. Another possibility could be that you need to update the chipset drivers.
But it's not a good burner for DL media. So I would get a new nice one.
The Optiarc DVD RW AD-7240S series is nice.