My BenQ DW1620 has a tendency to get out of balance. Out of balance creates vibration, which would screw up both a burn and read back. The way I am able to bring it back into balance is to get it to spin at 16X speed. I use the speed patched firmware, which unlocks ripping speed. Before I burn a DVD disk, I through in a disk that I have previously burned, and then let Nero CD Speed to do a transfer test. Immediately after 16X speed is reached, I stop the test and proceed to burn with Nero.
Also, before I do a quality scan with Nero CD Speed, I do the transfer test first. That will get the speed up to 16X. Then I do the quality scan.
This procedure often eliminates the spikes near the end of a burn. Please give it a try. It doesn't cost anythig.