Change the water pump (every 3 years if possible). The impellers in the porsche design are known (99% probability) to break or erode. That's what I've learn on 986forum anyway. Results are very low water circulation and overheating.
Keep on the look-up for impeller debris in the thermostat area. And no, i am not joking. Ask porsche if you don't believe me... heard they gave away many FREE water pumps to nasty and pissed customers
I am surprised to see that even a 7 years old thread can still be indexed. Well spotted mate