Re: New girl on the scrounge ;-)
I don't think that's anything to worry about, it's just the thermostat opening and closing to keep the temp within bounds. When the engine is a little less than the ideal running temp, the thermostat closes to keep the coolant within the engine. Then when it heats up, it opens to allow the coolant to cool by passing round the whole circuit, including the radiator.
Factors like outside temperature, how hard you are working the engine and whether you are stationary, will all influence the heat in the engine. The thermostat regulates this.
Obviously when you start the car from cold, the thermostat is closed until the temp reaches about 95deg, then you will see it suddenly fall down to about 60deg as the thermostat opens and all the cold coolant from the radiator enters the engine. Then the engine gradually heats up and stabilises. As I say though, depending on driving conditions, it may open and close again, causing sudden but necessary fluctuations.