Don't be so concerned with getting a high burn rate. What would you rather do 'burn a coaster fast' or a 'good disc slowly'. Discs burned at lower speeds tend to be much more compatable with set-top players.
Are you able to create a disk image of your project (File>Create Disk Image)?
Do you have the latest Toast 7 update (7.0.2)?
Remember the Latin saying "Festina lente" - Make haste slowly!
For anyone else out there facing the same issue...I figured out a cheap way to solve my dual layer burn problems...
I used Toast 7.0 to compress my huge project onto a single layer disk. The quality was reduced slightly, but I was unable to discern the difference when I played the disk on my DVD player. So, until DL disks come down in price, I refuse to make any more $5 coasters. Yippee!
Multiplexor error. The disk failed to be written: error code 0x80020042.
Based on the rare experience burning disks other than Verbatim, that sounds like you're trying to burn too fast. I had that type of error using Fuji someone bought for me. Sent him home with those and used my Verbatim.
I recommend dumping the Memorex and get some Verbatim DVD+R DL. And use the 2.4X speed in Toast.
I've read here that ytou can create a disk image with the breakpoint and use Toast to burn it. Have not done that myself.