i got a 92 Mazda 929, two weeks ago i noticed that the car is being overheated, i took the car to a mechanic, he told me i need to change the fan clutch. we did so, and nothing changed. i took the car to the mechanic another time, he told me it's the thermostat. we changed it, and the car is still being overheated whenever driven for more than 20 minutes on a regular speed, knowing that the water pump is good and there's no leak at all. any ideas or anything that can help please??

Bad radiator. The radiator tubes are probably cloged with sediment and mineral deposits. Take it out of the car and have a radiator shop test it for proper flow. Does the car overheat only with the A/C on? Is the A/C auxiliary fan on when the A/C is running? Overheating can also be greatly contributed to by heat dissipating from the A/C condenser if there is no auxiliary fan to help it.

it overheats in both cases, but it overheats much faster when the AC is on, the car is now at some mechanic's place, he thinks that the fan clutch i installed is defective, we'll install a new one today and see if it will work or not, i hope it will work.
if it didn't the only problem left to check for will be as u said.

If your car over heats when at speeds above 25 MPH then it is not the fan, as more air is being forced though the radiator than the fan can pull thought it