[torontoworker]

I think you meant coolant temperature instead of oil temp correct? Unless your playing with the display hack and have the data from oil temp sensor displaying?

Changing the thermostat generally doesn't do much to control ambient temperatures once the car is up to operating range. Unless the internal diameter of the thermostat is larger then stock to promote a lot more flow you won't notice cooler running after 15 minutes anyway. In fact faster flow can actually cause issues as the coolant is flowing through the engine and back to the rads quicker (less restriction) with the coolant not remaining in the rad to cool off long enough.

One area that is overlooked quite often is the overflow bottle cap. It's hard to detect if it is functioning properly until it really fails and you see coolant dripping out of it. When they fail it is sometimes so gradual that you don't notice it right away but it is allowing a pressure leak and this alters the boiling point of the coolant. It doesn't take a but a few psi lower then required to affect the cooling and stop and go traffic is where you will see it first as you don't have the airflow through the rad's to help offset a temperature rise. As soon as you start moving and get away from other vehicles you notice the temps drop right? So your coolant system is on the edge here and *something* has changed its state.

Do you hear the fans change speeds (two speeds) when the heat rises (over 225F I think) as your stopped? Perhaps you have a faulty fan or temperature sensor?

Verify the coolant temperature independently - the thermal scanners are under $50 at auto parts stores - or you can do the temp hack for your climate control unit depending on the year of your 996.

Water pump issue? Those plastic blades break off causing other issues down steam that we don't want to think about but these suckers are a wear item and not a lifetime part. Just saying.

I would also check to see if the rad surface is clean as well. If the cap *was* leaking, make sure you burp the system as any air in there will again raise the boiling point artificially.

But... always do the cheap / easy things first (the cap is$20-$30) before going to the expense of the 3rd rad or other non stock changes or throwing parts at the car. Make sure you get the right cap: Latest part update for coolant cap frpm Porsche is: 996.106.447.04