It's as bad as 1mm or 2 below 80C on the gauge on the highway. I am thinking of getting a new thermostat too.

I had the same thing happened to me last April. Driving in highway temp was below 80deg. Then all of a sudden temp was about 95deg, i had to turn on heater to make sure temp does not go any higher. Had car checked, it was the thermostat. After changing it, temp is always at 87deg (Day or Night)

You may want to check your thermostat.