Temperature display at high outside temperature incorrect

Hello to the round,

I was last week with my fat guy in Tuscany... in 10 hours at a time nice and nice 1000 km and it was running like clockwork. I only noticed that when I was driving in Tuscany (usually between 29 and 35 degrees on the display in the car), the cooling water temperature suddenly dropped. The display has always come up on 90 (as always) and then dropped off again... once to 70.. 30 times... And now and then, not at all. Did I notice on three trips... on the return trip the same game, when it became colder (I noticed it from 23 degrees and less) everything was stable again at 90 degrees...

Do you know that? Can / should I exchange something for security? Of course I have cooling water etc. directly looked (standing overnight and right after driving), everything was fine... Today, on the way to work (180 km) also remained stable at 90 degrees. I noticed this in DE (one or two times) but I never thought about it and didn't notice the connection to the outside temperature...

Do what?

Greetings.
--God