I am guessing that you do not have a closed cooling system and the current water temp is pretty chilly. As long as it stays around there, I wouldnt worry, its just the thermostat opening and closing. I notice it in my skiboat, especially when the waters like 50 degrees. The boat heats up and when the thermostat opens, the water is so cold that it cools the motor down quite a bit and then has to heat up again. I am also NOT a mechanic if I am wrong.