After driving for around 2 hours the car will occasionally stumble for a few seconds when leaving a stop. It clears up with throttle applied or when revs up over 2500 rpm. It never does this during regular driving (less than 2 hours) such a mystery as the car runs very smooth and strong otherwise

 

I suggest using an obd reader to check the cts and iat sensor values when cold (overnight) and after your extended drive to see if they are displaying "peculiar" values that are higher or lower than logic dictates. Those little buggers can wreak havoc.

 

OBD will likely call cts ECT but good suggestion.