I don't think this is too uncommon for an Auto, my 2006 323i did it a bit when slowing downhill at around 2500 revs, and it annoyed my wife no end. Obviously the car is changing down gears to slow the car, and my guess would be that the extra load the AC puts on the engine is causing that to occur sooner, or more dramatically.
Maybe changing driving styles could make a difference, try braking harder, or braking softer, or not stopping as often
Or take it to your dealer and get them to check it out, if they say all is OK, try and take another 1 for a test drive to compare
My Father-in-law had a Citreon, and it was a real b#### for it.