You may not want to hear this... but you may be driving it out of its power curve... this car was made in " the land of no speed limit autobahn' .... and any car needs to be in a certain rpm area not to overheat. IT IS A EURO... not built to be sent to the US.. you may just not have the Asphalt out in front of you to utilize 5th gear.. without changing the rear axle ratio....
Your car might do just fine in 5th at 95... but how long will you still have a license here in Texas if you practice that often ?
I assume that because you checked the injectors that you know your fuel timing is right on the money... on gas engines the timing of the ignition will affect the temp quickly...
Because I consider this a problem of efficiency.... I wonder when the little holes in the precombustion chamber were checked for cleanlyness ? Holes clogged can narrow your acceptable power band.
On some engines I have seen in the past... the water pump had FEWER vanes on it when the car had air conditioning... I was really surprised because that is opposite to my logic.. but it was something like" to keep the water in the air stream longer" before it got back to the engine....