the coolant temperature goes from normal operation temperature (before the half way line) to slightly above half (sometimes more)

This thing has been going on for a year now, but every mechanic has told me not to worry, yet non were convincing!

What happens is that the coolant temperature goes from normal operation temperature (before the half way line) to slightly above half (sometimes more).

This happens when stopping or slowing down while the AC is off during summer nights only! it never happens when the AC is on.

so obviously it might be fan related (or is it?) and it is probably due to the fans turning on at above the half way temperature, but is it normal to reach that point? I feel that it is not, especially because my friend's Malibu never does that.

note/ my engine light has never been on and the picture shows the operating temperature of a Malibu.

Thanks in advance