OK, here is a new set of recommendations. Since the thermostat is new, let's assume it is OK. Next let's look at those things that can affect cooling. Good maintenance practices say that the radiator should be clean and the coolant is the correct concentration. Flush the radiator and install fresh coolant. Recommend no more than 50% concentration. More water means better cooling. However, get the concentration too low corrosion can be a problem. It is best to stay above 40% unless it gets very cold in your location.
Next, get a garden hose and thoroughly clean the outside of the radiator. This will ensure adequate air flow.
Verify the fan. Let the car idle until at operating temperature. Unless it is very cold outside, you should observe the fan cycling on/off to maintain engine temperature. While you are at it, verify that the fan turns on when the AC is on.
Check to see that the radiator cap is holding pressure. Is the upper radiator hose rock hard at full engine operating temperature? If not, suspect that the cap is not holding pressure or there is a leak somewhere.
Now we know that the system is clean with the good coolant and in good working order we can get down to brass tacks.
Get your ODB-II scanner and data log the engine coolant temperature. This will give us actual numbers to work with. Also log the TPS and engine load parameters. We can then see if there is a correlation between engine load and engine temperature. This will also help determine if the cluster gauge is at least reasonable.
If your scanner also allows it, log the fan as well. We can then see if there is a correlation between engine temperature and fan operation.
As an additional cross check, an infrared temperature scanner is affordable. Use the scanner to double check the temperature numbers reported by the PCM.
Let us know the results. Your stang will love you for it. Good luck.
Your ride is fine.
