A few months ago I had a coolant leak and had my coolant housing assembly replaced which including a new thermostat, the thermostat was faulty and my car was not getting up to temp. The mechanics replaced that free of charge and things were supposed to be good to go.

Since then my car will get up to temp now but it takes way longer than it used to, right now its summer so its not as bad. Then this is where it gets really weird; if im going 25-40 its fine and stays at temp, if im going highway or express way speeds it’ll fluctuate a little bit and when i drop speeds my temp will also drop and sometimes wont get back up to temp even if my speed goes back up. Mechanic said this is a very unique issue and he has no ideas, only looked at it for about half a day.

I was told by another mechanic that i may need to get it burped as there could be bubbles in my coolant system causing the fluctuations. Got that done and it didnt help at all.

Ill be taking it to a third mechanic soon but my hopes arent high and I know theyll have it for a while. So reddit mechanics, without looking at my car, have any of you heard of a similar issue or have an idea of what could be going on?

Second question, is this a big problem? I understand how my car not getting up to temp can be bad but if its at temp or near it 80% of the time is it probably fine?