Try running "PlexUTILITIES" and check for problems. Offhand I am inclined to think that the write speed for the lightscribe part may be too high, or some type of movement of the disk is taking place during this phase of the write process. Not sure whether this can be adjusted or optimized.
What does Plextor have to say regarding this? Mention possible disk movement (during the lightscribe phase) to the Plextor Gods when you contact them.