Just a thought ... You may have a head gasket issue - pray you don't. Here's a step by step set of instructions.
1. With the motor cool, check the oil fill cap. If the underside looks like a chocolate milkshake, you have water in the oil - very bad.
2. Take the overflow tank cover off and check the coolant. If it's green, you're good (the more neon looking the better). If it's brown, it's bad. Go to step 3. If you see nothing, you have a leak or a really bad head gasket. Don't run the motor until you fill up the system and bleed out the bubbles (I think your car will bleed itself automatically).
3. Turn the motor on with the overflow cap off. Watch for air bubbles - bad sign if you see them = head gasket.
Here's what may be happening. Hot exhaust gasses are being pumped into the cooling system via the leaky head gasket. At speed, the cooling system may have enough thermal capability to cool the motor, but at idle, the pump is turning slow and the coolant will have hot spots. Even with the electric fan trying to push air through the radiator, it may not be enough to overcome the heat coming through the head gasket.
I hope I'm wrong.