My car is fine during the winter, but during the summer (we're talking Alabama summers here, sometimes 110* before you account for the heat index) my fans will have to come on and run hard if I'm sitting still. I'll probably replace the thermostat soon, just because I'm sure it's never been done, and I need to find a slow leak in my coolant system while I'm at it.

 

It should be near the opposite if it was stuck open. It would take a long time to heat up - but should act normally after it reaches operating temperature.

The temperature will drop when driving as it does when the fans turn on.
I would still change the thermostat and rad cap as a part of regular maintenance.

 

Your problem is not a problem at all and I think you're missing that point as it has been said several times. The middle of the gauge is normal. 3/4 up the gauge is a little high, but still not at a point where you need to worry. If it is getting in the red, then you have a problem.

The reason your gauge rises when you slow down is because there is little airflow through the engine where else at 60 mph you're shoving that air through it. Depending on your climate this time of year, chances are, your fans won't ever kick on while sitting still because the outside cold air is helping to cool the engine before the coolant can get to the temperature required to kick them on.

If the gauge isn't in the red, and is sticking around the middle part, your car is acting just as it should, no need to worry.

 

Normal operation of a car sitting at idle.
Car start, Coolant can flow through the heater core only at this point. It will run this way until the temp reaches 195 degs. Then the thermostat opens and cool coolant flushed into the motor. Depending how cold it is this can cause the thermostat to close again. Once all the coolant reaches 195degs the thermostat will stay open. The tempter will continue to climb until the fan kicks on around 225 degs. The fan will run until the coolant drops to about 205 degs then it will shut off and the cycle starts all over again. On a real hot day it may stay around the 225 until you start moving. (This is normal!)

Normal operation of a car Driving
While driving there is a lot of factors that make up how hot your motor stays. The speed of the air passing through the radiator, the tempter of that air, the strain on the motor. How well your coolant system is working (Crap in the radiator). On a cold day here in PA I bounce off my 180 thermostat on the highway. I am sure it needs to shut down the flow of coolant at times to keep the motor from getting too cool. Fans are only designed to work when the car is not moving or moving very slowly so as you come to a stop the above at idle cycle start all over again. So today I expect my temp gage will read around 180 degs while on the turnpike, if I have to stop due to traffic it WILL go to 225 until the fan kicks on.

For every one HP you can makes your coolant system needs to deal with one HP worth of heat waste. So things like pulling hills or running it hard should and will cause temps to rise. When it is 110 degs out that air will not absorb as much heat and that will cause the temps to raise, lastly don’t trust the factory gage. They suck. Get a scanner and go into the computer and see what the computer sees.

Leak should always be tracked down. If a small leak is letting air in the system on cool down it can cause a lot more problems than the quart of coolant you need to replace. Air pockets can do damage. You system must be able to maintain pressure to work as intended. Your coolant may never get over 230 degs but there are hot spots around the heads that get hotter than that. Without pressure these areas will boil and cause air pockets.