Mine used to sit around the 2nd notch on the highway nice and cool then get up to a 1/2 in traffic then the thermo kicks in and cooled it back down to half.
Then after a round at the mechanics it started going up to 3/4's then coming back down to a half in traffic. On the highway it'd come down to about a 1/3rd.
Then the heater tap blew under pressure and the temp was sitting around 3/4's when the water dropped out on the highway, then the motor cooked and was basically stuffed.
So I found a donor car for $500 checking the engine ran good, and I got a good one. I replaced all the hoses, got a new heater tap, etc and did an engine swap and now the engine sits back around the 2nd notch on the freeway and only ever gets up to a half.
It has gotten up to 3/4's once and I thought this was serious because it really should never get that warm. It puts extra pressure on the heater tap and you'll get into strife like I did. That was a really hot day too (Brisbane Heat) during the heat wave and even the power steering overheated that day but haven't had any problems since. I also flushed the power steering fluid after that.
When your car is running at 3/4's, it takes away the feeling that there's something wrong and this could happen to you on the freeway, so whatever it is, get it fixed so that way you can panic when it actually happens.
It's possible the heater tap or thermostat is causing problems, so I'd replace both and get a whole set of new radiator hoses and heater tap hoses.
When replacing the hoses at the heater core itself make sure you cut them off carefully because pulling on these hoses can damage the heater core inside or pull /rip the fitting on the heater box.