After doing all the above that people talked about you might have a plugged radiator, if it heats more on an uphill pull that will be the cause. You can get a radiator shop to pull it apart and rod it out, but that sometimes cost more than a new radiator. Another cause we see is a bad head gasket that lets in pressure every time a cly. fires. Most shops have a snuffer that will turn color when combustible gases are in the coolant. I've also seen the impeller come lose on the water pump and under higher RPM just spins on shaft.