I've heard of this but I experienced it myself last week. In 6th at motorway speed, a bit more throttle and just for a second it was like hitting a rev limiter before continuing as normal.
I usually use BP normal 95 unleaded but refuse to use the expensive stuff, I rarely use supermarket petrol mainly due to the enormous queues caused by drivers trying to save a couple of pence per litre, so I'm sure it's not because of poor fuel.
The car is due a service in a couple of weeks and they did say it was due two updates but I didn't think to ask what they were for. I will let you know the outcome.
Edited by Avon, 08 August 2018 - 03:24 PM.