Whoa this is a old thread that got revived. Anyways when you're accelerating and RPM increasing, so is your water pump and oil pump is also pumping faster pushing cooler fluids through. Air is also rushing through your radiator to cool it down.

This is normal. Once you're at speed and let go of the gas, the engine is producing less heat, while you're still moving air is pushing through your radiator. Thus cooling more. There is a limit, as the thermostat closes it won't get any cooler.

Hope that helps.

 

Air in the cooling system can also cause temp gauge fluctuation. I believe the shop manual for Cayenne's recommends the cooling system be pressure bled as opposed to gravity.