[ERA Chas]

You have air pockets in the coolant-the gauge spikes because the air is hotter than the coolant.

You must get all the air out by opening the system (cold) at the highest point (usually the temp sensor on the intake) and filling until the coolant runs outs of the sensor opening. Put the sensor back in, then slowly add to the overflow tank but leave room in the tank for expansion-do NOT fill to the top.