Try replacing the radiator cap.

Sounds trivial and stupid, i know... but I was having the exact same problem with my eclipse. At a stoplight, or just idle, the car would begin to overheat, but it would quickly subside as soon as I revved it or began to move.

Turns out... the radiator cap was leaky, and the cooling system was losing pressure and some fluid. Easy do-it-yourself'er, or 20 bucks at pepboys. Worst case scenario, it isn't it, and you have a pretty new cap.

yeah... air pockets will definitley make that difference... screws up the pressure gradient needed to keep the fluid moving..