Some things to know about cooling systems.........when running a water based coolant, when the motor gets hot, cavatation can form in the pump and in the block. That bubble will move eventually, that' why you will see the temps start to rise, only to fall fast due the bubble being broken. Another thing is the so called "dwell time" in a radiator, Any time coolant flows through the radiator it is being cooled. The temps of the coolant coming back through the suctions side of the pump is what is important. If the fan, pump and rad are doing their job correctly, then the coolant coming back should be 10-15 degrees cooler than what the gauge is reading. Lastly, the sending units for temp gauges are usually at the front of the manifold, right near the thermostat housing. So the coolant temps are being gauged as the coolant is LEAVING the motor, not what the temps are in the motor.