On the above car I have noticed a lack of willingness to change up in gear.

This only happens periodically and always when changing from 3 to 4 and sometimes 4 to 5.

In drive 'D' the car when accelerating normally will change from 3rd to 4th at about 1850 rpm and similar for 4th to 5th. However sometimes it will hold onto the revs upto 2300 rpm before going to 4th from 3rd. It does not matter if the car is going up hill, down hill or on the flat. The engine is also up to temperature. I also have paddle shift, if I force a change in 3rd at 2000 rpm into 4th it changes no problem and holds there until it goes back into auto mode and then it reverts back to 3rd gear again and will hold until 2300ish rpm. In sport 'S' mode the change happens from 3rd to 4th at about 2500 rpm so much higher so no issue about either me or the car being in 'S' by mistake.

This is really annoying as it only happens occasionally. If you stop the car and switch it off it will re-start ok and behave.