I'm having an issue with my 2012 S4 (64K miles) and I'm at my wit's end about it. After driving the car for a while (or when it gets to operating temperature), if I stop for a few minutes like when in traffic or when I park, the engine fans turn on at high speed and the temp needle on the cluster rises above the halfway point--but doesn't reach the 3/4 mark--for about 15 seconds, then goes back to normal.Back in December, my water pump went out and my mechanic not only changed that, but also changed the thermostat. This time around, they had the car for about 3 weeks and have changed the thermostat two more times, the water pump again, the radiator, various hoses, and bled the system ad nauseam. Still, the problem persists. It's also not losing any coolant. My mechanic also told me that when it overheats, it seems as though the coolant isn't flowing smoothly, but after changing all those parts, they have no idea where the blockage could be. Has anyone ever had this problem or have any idea what additional steps to take to troubleshoot and/or fix it?Some potentially relevant info: the car has a Revo stage 2 tune and an ECS Tuning intake. But those have been on for a while before it started doing this.

Thank you for your help