I also have a 111D flashed to 111L by the TDB 8.19 firmware. Both before flashing and afterward, I've consistenly seen it go to 0 speed right around the 1.2 GB mark on Verbatim MKM001 DL media, whether recorded at 4X or 6X. This is supposed to be CLV all the way at either speed. The strange thing is that according to ImgBurn 220.127.116.11 (which I'm burning with), the device buffer doesn't seem to empty when it does this, so it soesn't appear to be related to a recalibration/relinking!
What's more disturbing is that when I examine the surface under a bright light, I can see a thin shiny "ring" out about 30% of the way from the hub! I assume this is related to the spindown at 1.2GB as this would be about the right position on the disc on layer 0 of the media.
This has happened on 3 DL burns now (all on Verbatim MKM001 media), but the quality check comes out great regardless, with no quality issues evidenced anywhere near 1.2GB.
My Plextor PX-716A doesn't have these issues at all and I get a nice solid burn all the way, as evidenced by the DVDInfo Pro graphs produced by ImgBurn. I've gone back to my Plextor for DL burning, even if it is only because I don't like the aesthetics of seeing a ring on my Pioneer DL burns.
In addition, the Pioneer speed does jump around a little throughout the entiere burn, while the Plextor stays rock solid!