Thanks for your replies.
The car has had this problem since I bought it a year and a half ago. I have not replaced the coolant but I believe the radiator was replaced about two years ago. Surely the system would not trap air for that long, would it? If so, what part of the system are you talking about bleeding?

My temperature gauge seems to work fine. Is the ECT sensor the one that sends a signal to this gauge? If not, can you point me to it.

Thanks again