DB9 auto erratic gear changing?

[cavendish]

Just been for a nice leisurely drive in my DB9 and noticed that the gear changing was a bit erratic at speeds up to around 45mph when in drive. It would go into one gear and then change down and back up again with no real change in speed. It is fine at higher speeds and on the paddles
To me it feels like an automatic that I had many years ago that was slightly low on auto gearbox fluid and when the fluid was stopped up it was fine. Is the DB9 likely to be a similar problem and possibly just a bit low on fluid? Had a quick look to see how to check the level in the auto box but have drawn a blank so far.
Have any other owners had a similar problem and can the auto box be topped up? My car is a 2005 model.
Thanks