I wasn't referring to the radiator, I was referring to the heater matrix (cabin heater)
If they're connected the wrong way around, it could cause an air pocket that can't be bled off properly.
Looking at a diagram in the manual, the heater hoses cross over one another on their way to and from the engine, so it would be easy for someone to get them arse about.
Just a thought; you say the TF is a 2003 pre PRT? If a PRT and standard engine block return thermostat were fitted you would get the same symptoms as the heater circuit by-passes the standard thermostat back to the block allowing coolant flow; albeit a tad restricted, when the heater valve is turned off flow will stop; no matter what temperature the engine gets to.
Have a good look at the coolant circuit and check for a retro fitted PRT (an alternative position for a retro fit PRT is at the front subframe)
Brian.
I'm presuming that the only signs of of overheating is a rise on the temp gauge. If so my thoughts would be an air bubble around the coolant sensor area. Opening the heater circuit may be enough to move the bubble away from the sensor returning the reading to normal.
I have had this happen to me, and carefully loosening the engine bay bleed valve allowed the trapped bubble to be expelled. Beware hot coolant.
Do you have an OBD reader and torque app? This will show the temperature that the ecu is reading.