Engine racing occasionally

[Steve the salamander]

Haven’t got a clue here.
The engine of my car - 2013 Citroen C4 Picasso - occasionally races for a few seconds when driven at 65+ mph and informs me to put it in 6th gear when it’s already in sixth gear. No engine fault lights come on. Garage has done a diagnostic check but can find no faults. Anybody got any ideas what may be causing this?

[GiveMeABreak]

Are the wheels / tyres the correct size for the vehicle, as in originals and not been changed for Carlos Fandango super wide wheels or the like?