Hi.
what is the engine temp gauge saying when the heater is blowing hot/cold?

A problem I had with my TDCI mondeo with very similar symptoms turned out to be the oil thermostat which is located on the back of the waterpump.
My temp gauge would stay really low when driving and would raise for a while if i stopped for a while with the engine idling. My heater followed whatever the gauge was saying.
I tried changing the main engine coolant thermostat first as it is the easiest to do.
This didn't fix my problem, so I then changed the oil cooler thermostat which did.


Hope this helps,

Nick