Cavalier running hot

I have a 1997 Cavalier 4 cyl. With 170,000 miles. The car runs hot ( almost overheating) at highway speeds while maintaining good interior heat. At speeds below 30 the heater sometimes blows cold air. If the car starts to overheat, I can shut the blower fan off or on low and it usually starts to cool off. The only coolant that I can find leaking is from the reservoir cap (the car does not have a radiator cap) when it starts to overheat. The water pump and thermostat were both replaced about 35,000 miles ago. I'm adding about 3-4 cups of coolant after a 70 mile round trip. I'm at a loss, please help.