in case you have a blockage on your radiator it is going to abate the flow of fluid, consequently combating it from cooling. At larger speeds what flow is occuring is getting extra airflow allowing it to sit down back extra. It in all danger isn't your thermostat because of the fact that could block all coolant flow and reason it to overheat at any velocity. It overheats swifter while working the AC given it is extra taxing on your engine coming up extra warmth to get rid of. Worse case state of affairs is which you have a blockage interior your motor, wherein case you have a real concern. something you're able to do at domicile for a rapid verify is while the motor is at idle and warm is to squeeze your hoses popping out of and going into the radiator, (use a rag over your hand, the hoses would be warm). If one is demanding, I recommend relatively demanding, you have a blockage in it. do not forget that your decrease hose, often yet not continuously, has a spring in it to keep away from it from collapsing from the suction from the water pump. for this reason it could have some supply to a element. Blockages can ensue from a enhance of outdated antifreeze or a deterioration of the copper interior. i could start up with a intense rigidity radiator flush, it could reason your radiator to fail whether it is corroded. even though it is extra advantageous for it to fail in the shop then on the area of the line. If that doesn't restore it then i could have the mechanic evaluate it, in case you believe him, he shouldn't steer you incorrect.