I encountered it shortly after getting the 720 also in my bubble canopy Grumman AA1B on a 7 hour round trip flight from the Bay Area to Lost Wages (Las Vegas).  It probably did it 5 or 6 times that I caught.  I did not fly the 700 enough in the Grumman with the 7.5 load to say it did not happen there also.  I also had just started flying using the Portait mode with the unit mounted on the yoke.  My Vegas trip was under very smooth flying conditions, and I caught it happening when there was no movement of the yoke.
Love the portrait mode by the way.....