Belt tightness, fan clutch, system seal (no air leaks like from heater valve or radiator cap pressure relief or "cracked top radiator tank around hose connector or parallel to front or rear of top plastic tank), thermostat failure (my experince with other cars rather sudden and not remedied by cooling off engine by shutting it down and running it later but is possibility). Fan clutch symptom - car runs fine at speed but heats up when stop and go required and least likely to be detected by movement of fan by hand.
