Almost all temp-related issues seem to be the thermostat. Even a brand new one can be bad and not open fully or at the correct temp, so don't think that just replacing it will solve eveything, ever. That said, every time I've had the temp gauge go higher than midway, or when the system has nearly overheated, replacing the thermostat has solved it. Apparently when the engine temp goes much higher than normal, the wax in the thermostat that causes it to open and close is permanently damaged, and so it no longer functions correctly. Be sure when you get a thermostat that it is the correct temp range, 190-195 degrees. Crappy thermostats, by design, allow more temperate range and some even let the gauge go wayyy over to the right before opening and allowing coolant flow. The ~$20-30 Motorcraft 'stat is about the only one that opens and closes constantly to regulate temp and keep things at ~190 degrees. The $5-7 Stant Superstat is ok, but still allows some temp range. The thin and flimsy cheap 'stats allow a lot of range, but can be good as a temporary fix.
Also a lot of rust/corrosion in the cooling system, or blockage in the radiator and coolant passages will cause heat issues as it won't allow the coolant flow needed. There are other things too like a busted radiator fan clutch, or if the sensor on the fan clutch assembly is no longer doing it's job, then you get a cooling fan that just freewheels or doesn't engage correctly to move the air necessary.
Air in the system does create problems, but from what I've seen, it's almost unnecessary to 'burp' the system if you slowly refill with coolant over an hour, giving 5-10 mins for it to settle before filling again until it's within an inch or two of the top, then put the cap on to the first click, run the engine for ~5 mins, then top off, fill the reservoir, take it for a spin, and then top off the rad and tank once it's reached operating temp at least once to let the 'stat open.
