over heated engine and heater blows cold air

[jictat]

I have a 1995 ford mustang 3.8, my problem is, after two or three miles of driving the temperature, the indicator passes the place where the thermostat opens and the fan runs then shows that the engine is overheated and the heater start blowing cold air, I took the car to the mechanic and he changed the thermostat and replaced the anti freeze, I drove the car again and it was the same the indicator goes almost to the end of normal, (never passes to the red line), heater blows cold air and after couple minutes it goes down again and works fine, thermostat and radiator fan works etc, heater works, is only after I start driving and the car warm up, I'm concern because I don't know what's goin on, and because the mechanic couldn't solve the problem.

I checked the engine when it happens and it doesn't feel overheated

Please help me.

Thank you