2003 Audi A3 Engine heating when car is stationary

Dear sir,

I have a nearly 6 mth-old problem that my local mechanic cannot seem to resolve. Here is the problem:

The engine temperature gauge continue to spike to 100 deg C (normal 90) after the engine is cut off (parked), causing the 2 fans to come on and cool the engine. All this may seem normal - but I have not experienced this over short and calm trips to the nearby grocery shop, until half a year ago, when my water pump broke down (causing the engine temperature gauge to spike). I stopped the car, had it towed back to workshop, replaced both belt and water pump at that point.

Since then, no breakdown as the 2 fans faithfully comes in to cool the engine, and no other symptom has happened so far: No pressure leak has been detected, and cooling liquid has not dropped.

Here is what I have done to try and troubleshoot:
1) Done 2 x AutoRx to remove any sludge suspect in engine, replaced w/mineral oil (30 grade) and change every 5000km or 6mth interval.

2) Replaced coolant liquid

3) Pour in 1 bottle of fuel treatment in gas tank to remove any water and improve fuel efficiency

4) Replace temperature sensor to the fan

As far as suspect list goes, I am down to either:
1) Replace radiator
No leak in water tank, so I am not sure this is a viable suspect

2) Replace water pump
Water pump should be working alright since engine temperature when moving is stable, not fluctuating.

3) Replace fan(s)
As an aside (don't know if this is related) : Sometimes the aircon does not feel cool, and gives off a stale smell, which changes once I shut down aircon or change temperature within the car.

But I am not sure: this issue should not have caused the car engine to be hotter, since it is not really dependent on fan to cool. (The fans only comes on when I start the aircon, or if the engine becomes very hot, ie. Travel faster during trip without aircon, fans will come on).

Please help. The car runs normal in all other sense of the word, I just completed a 500km trip, and at the end of the trip, the temperature only threaten once to spike while the engine is running, but retreated once I turned off/on the aircon.

Regards,
George