usually has to do with the thermostat, mine does it also, you can either fit a dual acting thermostat or drill a small hole in the side of the existing one. I say next time you give it a service, change the thermostat and coolant, should get rid of it.
In case you wanted to know its caused by the thermostat blocking the majority of the coolant, one side is warmed by the engine and the other is dead cold, once the thermostat opens you get a rush of cold water coming through, Make sure you fit the correct heat range too, i think its 98 degrees?
Lets go find that Hotdog tree I planted
Lowered EF Futura
Black Steelies ( too cheap to buy proper mags )