My 86 GT has been overheating once sitting at idle or traffic after a short period of time. when i drive on the highway the car doesnt run hot at all and this is in 70 degree weather. i dont know when the coolant has last been flushed but it looks ok. i dont know if or when the thermostat or water pump has been replaced. Any simple things i can check for? The cooling system is completely stock. Thanks for any info?