i had an rs turbo van that did this, in that instance it turned out it needed a new radiator. when the turbo came in and put the engine under more strain rad couldnt cope and temperature went up. if your rads blocked or collapsed inside it dont matter how much air you push through it it cant cool the water. feel all over the core when the cars hot and see if the core is hot everywhere.
if everything youve said is correct and you got no pressure or oil and water mixture issues im fairly confident itl be your rad mate.