2002 Audi A4 Mystery Overheating Issue

My Audi's temp starts climbing when I'm at idle, but it never goes all the way up (around 3/4). As I start to drive, the temp slowly goes back to normal. I can also rev it up, while in neutral, and it will go back to normal. The waterpump and thermostat were replaced less than a year ago, but just to make sure, I replaced the thermostat and the waterpump looked good. Nothing. I replaced the radiator. Nothing. I checked the fan and fan relay. They both work fine. I'm not sure if this problem was related to only getting heat through my cabin vents at high speeds (freeway), so I bypassed the heater core. Still the same problems. The coolant level remains full so no cracks or leaks in the resevoir tank. My mechanic and I are stumped. Any help or ideas are appreciated. Thanks!