Re: 87 300D Odd Temperature Spikes

Normal. But your concern about overheating this engine are well founded.
Whenever ANY engine is shut down and soon restarted its apparent temperature will be quite high for a brief period. Normal. That's
because the coolant is at its hottest AFTER the engine is shut down - the flow stops and the now stationary coolant must absorb all the heat that's in the head (mainly) and block. The sensor is on the thermostat housing, so upon restart all the very hot coolant passes by it enroute to the radiator. Next time you buy fuel look at the temperature gauge upon restart and you'll see how it rises and then quickly falls as the gallon or two of hot coolant is moved into the radiator.
Charging a hill and allowing the car to decelerate as the slope increases saves a bit of cooling ability but it's not significant. It may however ensure that the oil cooler's thermostat opens at 195 degrees F and that the oil cooler is operating when otherwise it may not be. Check if the oil cooler is too hot to touch on your next hill climb; its job is, of course, to cool the engine oil and so shed heat from the engine. Be sure its core is clean and not blocked by grit, bugs etc.
You've done the right thing in replacing the radiator and coolant - these can be eliminated as suspects. An don't worry if the engine's temperature rises during a hill climb; it's normal so long as the temperature falls back to 80 degrees C. when the road levels off.
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