Probe location sure will effect the readings. The back of the engine sees less water movement till the thermostat opens, since moving water will take the path of least resistance. If you can circulate water, then the spike will go away. I relocated the temp probe on my 250 to the port right under the thermostat instead of the cylinder head. Mine would spike just as yours is doing. My heater hose has a vacuum shut off valve to stop flow to the heater when not in use, so the small bypass built into the cooling system wasn't enough to really keep the temp even across the head. Once the thermostat opened, the engine stayed steady at 180. I no longer get the spike and I know just what the temp is leaving the engine.
I figure as long as the temperature leaving the engine is right, I don't really care what it is doing back in the head or block.
