What is wrong with my car?

My car will not stop overheating. It does this thing for about a year now, where after you've been driving a while, it overheats when you slow down. But if you keep it going at a speed above 70, it cools down. But the second it drops below that speed, the needle shoots up to the H. I keep putting coolant in,... show more My car will not stop overheating. It does this thing for about a year now, where after you've been driving a while, it overheats when you slow down. But if you keep it going at a speed above 70, it cools down. But the second it drops below that speed, the needle shoots up to the H. I keep putting coolant in, I've changed the oil, changed out the thermostat, I've taken it to several mechanics, several times, and they never can find an issue, even after spending hours inspecting it.
Also when it overheats, I try and relieve some of the heat by turning on the heater, but it just blows out cold air. I don't know what to do. Any information would be great.
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