I am trying to diagnose an issue with my Mk1 Golf. The engine heats up when the car is idling or in traffic but cools down almost instantly when I start moving again. I would presume this is the electric thermo fan not kicking in but it is working. I'm assuming the pump is working too as the car cools when driving, so I can believe liquid is circulating.
Could it be a clog in system that limits flow when the cars idling? Would anyone know what could be causing this?