Overheating

Hi,
When I start the engine in the morning and everyday I drive four and five hours in the morning. There is no problem with the car after four or five hours driving it starts acting up. The temperature goes up.I replaced thermostat, cap, all connections of the fans, crank shift position, water pump, but unfortunately mechanics do not know what the wrong is with this car. The oil level of the engine is good. No leaking, no mixture of oil and water, no low coolant, nothing. When temperature goes up I turn on A/C and the gauge stays under hot and whenever I speed up the gauge goes up and when I move it goes down. When I turn the heater on there is no hot blow, but when I accelerate the hot air comes out. I am really really fed up with this car. I do not want to spend more money. This car is really fuel efficient but I could have bought a Lamborghini if I did not buy this car. Please help me if know what is happening.
Thank you so much.