With my old faithful 981 this is a very common phenomenom. The pattern has changed between firmware updates, but they still persist. In some cases the egdes of the image have a saw-tooth shape. There may also appear short moments with zero range and no recorded structure at all. Also, the 2D view is recorded down to the range set for SI which means bad use of memory card space. I think the issues have to do with the way the unit controls things.
(However, and this is my standard complaint, these issues are trivial compared to the transmit pauses (causing recording gaps) which have prevented me from updating my gear. I will stick to my 981 until I know there is a unit/firmware that can do gap-free recordings. Note, these gaps are not easy to see since they don't appear like empty spaces.)
Rickard