Did you test the coolant with a proper anti-freeze tester to be sure the quality is good and it's mixed no more then 50/50?Also I'd try to bleed the coolant system there should be a bleeder valve on the thermostat housing assembly, open it up while the engine is running once you see only coolant coming out close it , do this more then once but try it over time too.Another possible cause is the fan might not be engaging try running the vehicle from a cold start once it sits at a warmed idling speed within a few mins the fan should engage if it doesn't it's probably the fan relay in the main fuse box under the hood or it's fuse or the temperature sensor or it's in the wiring or a faulty fan.
Hope that helps and best of luck.