at 128k mile,overheating which happens when the AC is on and am accelerating

Thanks for your response, the fan is working well, I will change the thermostat.

Did the thermostat replacement help? The temperature gauge on my Lexus goes up almost to the "red" line when I sustain speeds of 60-70mph for more than 5 min. The moment I bring the vehicle to a full stop, the temperature gauge drops to the normal position. It does not matter if AC is on or off. It does not usually happen on stop and goes in the city; only on higher speeds for more than 5 min. On longer drives it goes up to the red mark, then eventually drops a little (but still not to the normal level). I keep an eye on the coolant level; and not a ounce is ever short. I have replaced the temperature sensor, but still the same result. This has been going on for 1 1/4 years. Could it be faulty thermostat or a bad temperature gauge. It is nerve racking. I don't drive the vehicle for more than 20 miles away. Local lexus dealership did not know when I called.