Still is overheating

It overheats as im driving normally, within 10 to 15 min it is at about 260 degrees then when I pull over it goes down a bit and stays consistant. Then when I start driving again it shoots back up to 260 degrees. It has not been tested for combustion gases in cooling system. The parts I mentioned that were replaced by 2 different mechanics then the last one I went to for diognostics said water pump was leaking and needed new radiator cap. It was fixed and it is still overheating.

Have you bled the air out of the cooling system properly. Alot of the time people just bleed it by the thermostat nut. Try bleeding it by the bleed valve on the radiator and by the heater matrix bleed valve.

Also use a old drinks bottle full of water/coolant mix and put it on the header tank. If there is air in the system still you will see air bubbles come into the secondary header tank and the coolant will replace it. Do this while its warm and when the thermostat is open.