A sticking thermostat is usually a consistent problem and also affects overheating at highway speeds. If the problem is intermittently occurring at idle, an electric cooling fan relay is more likely the cause. (There are two
or three of them in the fuse/relay box). The fan motor, sold as an assembly or an open wiring circuit to the fans is also a possibility.
Chino Autotech Inc.,
Auto Club of SoCal Approved Auto Repair Facility