My 1987 245, 223K, overheats occasionally. The temp gauge needle will go past the midpoint almost to the red zone at which I turn on the heat and the needle drops. It happens either on the highway or going up a hill in town. AC is off since it doesn't work, which makes turning on the heater at 90 degree temps even more unbearable. The car received a new waterpump 2 years ago, new radiator last September, and a new thermostat 2 months ago because the engine was running too cold. Now, with the warmer weather, it seems to be running too hot. There are not leaks anywhere, and the coolant has been replaced 2 months ago. Engine itself has been steam cleaned a few months ago, so it has no crud on it. Any suggestions? It is due for its 3000 mile oil change.