No, I don't think you will have a ACT, the sensor for temperature input to the ECU is known known as the CTS (Coolant temperature sensor)

On yours it's actually a CHT sensor, it measures the temperature of the cylinder head itself, it's between plugs 2 and 3 (You see an electrical connector?)

Personally I don't think it's that.

It sounds like your cold start is ok, it's just the idle control circuit that isn't!

Martin