Hello, am having a similar problem on my Mk2 1.6.
Have managed to make it more consistent by ensuring there is no air in the coolant system. I did this by running the car with the bonnet up and coolent filler cap off until it ran hot and the fan came on. Once the fan has cut in u know the engine is up to temperature.
Ensure that it does'nt overheat by keeping water topped up. As the water circlates the system with the lid off it effectively bleeds the system of any trapped air.