Fiat punto heater blows cool air?

It is a fiat punto 2000 (mk2)1.2 16v.When I start the engine (car is standing) and wait till the water temperature guage goes to normal but the heater blows cool air. When I start to drive the temperature guage arm starts to going and lifting up and down between the zero and normal temperature, but dosent go over the normal temperature.Later the heater starts to blow warm air(I think it is not really warm but warm) and the temperature arm stays in normal position. Sometimes when I start the engine I can hear water gurgling. The headgasket, the water pump, the termostat, the belt changed 200-300 miles ago. Pls help me to find out whats happening?