Good day

In the past week I have noticed on two occasions my temperature drop from about 90 to just over 75/80 then creep back up in about 20 seconds

The first time was on the motorway, coming out of traffic that was crawling in parts, got up to 70 mph and the car seemed to drop to just above 80 then find its way back. I travelled 40 miles back in the evening and it didn't happen again. Today was I crawling through traffic and noticed that once I accelerated up to 40mph the temp gauge did the same.

Now the coolant is filled up to the max and I have a lot of driving coming up so want to know if there are problems ahead for me. The heaters are pumping out the heat, and the car takes a few minutes to get up to temperature of a morning

Let me know your thoughts