engine cooling system

What is your question?

While driving at high speeds, attempting to pass or accellerate my temp gauge starts to rise and if continued, it overheats. If I back off and slow down temp goes right back to normal. I have changed the thermostat, flushed the system. Thinking about clutch fan or water pump. No leaks.I can drive at 100mph with no problem but, if I punch it to pass, the temp will rise, if I back down to 80mph temp goes back down quickly.