Hello, I hope the following is of assistance.
There was a revised thermostat that opened at 74 degrees for this engine, check that you have the correct thermostat and it is fitted correctly (I have seen one backwards believe it or not) make sure the small hole is at the top.
If the car overheats at idle check the cooling fan operation. you mention as soon as you release the throttle the gauge drops so check the airflow through the radiator, make sure the fins aren't clogged by mud and debris, when up to temperature make sure the radiator is hot everywhere and that the top and bottom hoses are at the same temperature. Check the coolant expansion tank neck and cap to make sure there is a good seal as if the system can't hold pressure then you will get an overheat condition.
If possible read live engine data to make sure the engine is actually overheating and it isn't a concern with the temperature sender, there were concerns with the engine management control module calibration where the temperature indication was incorrect but given the age of the car it should be running a much later calibration.
Finally as the cooling system has been opened several times you must be certain there is no air locked in the cooling system as again this can cause overheating concerns.
Start with the above points and let me know if I can assist further, I'll reply as soon as I can.
I am taking it to my mechanic in the morning and will try your suggestions and get back to you with the results. cheers John
no problem, let me know if I can be of further assistance.