Do you not just think this is normal for the car? Whilst going down the motorway you are getting cool air over the rad, keeping temps down. As soon as you stop you lose the cooling ram air and so the heat-soak increases temps.
You know the fan works fine so imo it's just the lack of cooling air which makes the gauge move up.

For peace of mind you could fit a fan over-ride switch inside the car so as you slow down you can switch the fan on at a time/ temp that suits you keeping overall temps down.