Drops into lower gear for no apparent reason.

Hi everyone, I'm back with more misadventures. Lately my X-Type has taken to dropping a cog, usually when driving at low speed (30mph ish). It will do this for a short distance and then change up again. I did wonder if the gearbox was playing up (will be posting on that later). Is it possible that the car is going into dpf regen mode?  I do a lot of short journeys although this issue didn't manifest itself until I had a change of duties which has increased my weekly mileage. The car also seems to be running cool. The temperature gauge has never quite hit halfway but recently it's only clearing the quarter mark. Any thoughts gratefully received