Engine performance issues can't be ruled out when talking about engine overheats. Things like timing issues, both mechanical or ignition timing, can cause your engine to overheat. So don't forget to check the mechanical timing and the ignition timing. A restricted exhaust can also cause your engine to run hot or overheat because the engine can't expel spent exhaust and has to work harder as a result. This video can help with that diagnosis.
In addition to a restricted exhaust, a malfunctioning EGR can also cause an overheat. This is unlikely, but it has been known to happen. The job of the EGR is to cool the combustion chamber under periods of engine load. If it’s not functioning, it can contribute to running hot or an overheat. I will admit this is remote, however.
Also: low oil. If your engine is low on oil, it might overheat, so be sure to keep your oil topped off. Transmission problems also can cause an overheat, so don't forget to check its operation too. A sticking brake caliper can cause your engine to work harder and overheat. Basically anything that causes your engine to work harder than it should can contribute to an overheat; this includes towing beyond the vehicle’s capabilities. So if you just bought a yacht, don't tow it with your Golf GTI.