I'm with this thinking also. If you're driving around and it's fine then the thermostat is already open. Not sure how it could stick closed or partially closed at this point, unless it never opened fully from the start. If you reduce air flow, like sitting in traffic, it points to air flow components or a system that's not working very efficiently and only needs a little extra heat to cause problems, radiator partially clogged etc.Insufficient cooling at low speeds or when stopped (but adequate cooling when moving) suggests not enough air moving through the radiator because the fan clutch, fan itself or drive belt is not/no longer in the game. It is less likely to be a partially blocked radiator.
My E3 is doing the same thing so I'm replacing the suspect radiator and adding a pusher fan.