Thanks both of you for your suggestions!

I think the radiator cap is in good condition not only because of the visual inspections, but because I can hear the system despressurazing when I remove it.

I come to believe the problem was air trapped into the circuit indepently, and now after readings your comments I am somewhat more confident about it...

Now: How can I bleed the system? Should I replace all the coolant? I have some experience in motorbike mechanics, but not having a means to access the car from behind, nor tools with me makes everything more complicated, but I am considering to Do It Myself, just to be sure it is properly done.