I have the same problem with my 2002 Audi A4 When I am driving the temp gauge will stay normal. When I stop and sit idle for a few minutes the temp needle moves to the 3/4 line and the air conditioner get hot....I replaced the thermostat and that seemed to help but it is still doing it...ALso this seems to occur mostly when the outside temp is about 90 degrees..is this normal or could it be the water pump???

 

my thermostat is new "behr" my water pomp it's also new maybe at your type of engine you have an electric water pomp that start when you turn off your engine....must check this option