Gauge in the red, coolant resivor bubbling, and have already changed the thermostat

Have you checked for a bleeder valve on the engine? Often times on some imports they require you to bleed the air from the system so you get a complete puge of coolant. If you didn't do that then it could cause problems as you describe.

V6 or 4 cyl? Let us know what has led up to this. Is the radiator still the original? Any recent service work?