Hot engine problems

Hi, I have a mercedes 300ce24,1992, done 121000km, and when the engine temperature riches 90 degrees C, it is loosing power and specialy on a start from 0, I have to push the accelerator more than usual. It gets worse if the ac is working, and also in the traffic, where I stop and go more often.I leave in a desert where the temperatures and humidity is very high. Is there something I can do to fix the problem?In the local garages they do not see anything wrong, but I drive it and see the diffrence when is normal(at night). Regards.