Hello guys. First off all thank you for all the help you gave me 😉 . After i checked almost everything with my mechanic, and couldn't find the problem, i took the car (again) to the electrician, and he found the problem 🙂 🙂 . Well, as it seem's the car has a valve in the second radiator (the one that is inside and brings heat to the passengers). This valve over the years it can stuck close, half-close, open or fully open. In my case it stuck half close or half open, but it wasn't fully open. This valve controls the flow of the coolant that passes through the inside radiator. So, when the car was under constant pressure (@2000-2500 rpm in the highway), it couldn't have all the coolant flow it needed (it is just a small 1.4, so the highway is a bit hard for the engine, it needs to be cooled good). So the fix could done in 2 ways. The 1st way was to remove the dashboard and fit a new valve (and probably new inside radiator). Too expensive for the value of the car and my pocket. The 2nd option was to fit a bypass hose so the water could flow in a "close circuit" inside the engine, so the valve wasn't restrict the coolant. This fix was also recommended from the Ford dealers some years ago, but for the $$ they stopped (it is more profitable to change the radiator and the valve for them). So after i done this fix, the car runs like an angel. Smooth and nice. After 400km no problems. Also i have heat inside when i opened the heater, as i should. So a cheap fix (90€) fixed everything. If i was going the 1st way i would spend more that 350€... 😉 . Thanks for everything. 🙂  

it could be a failing water pump

What do you mean when you say the water is "boiling"? Do you mean it's bubbling like air is in the system, or boiling, as in what water does when it's on a hot stove? The coolant shouldn't boil like on a stove if the system is working properly, it should remain liquid. If it's bubbling in your reservoir like you're blowing air through a straw into a drink, then air is getting in there somehow -leaks, head gasket failure, failing water pump, loose hoses, drains, caps... Trapped air will cause the temp gauge to bounce around as air moves through the system, so will faulty fans. Turn your heater on max hot, and if it's not hot when the needle is in the middle of the temp gauge, there is air in your heater core, which means you're low on coolant. You may be losing it somewhere -leaks aren't always obvious. A few months ago my truck's radiator cap was a little loose and coolant slowly evaporated out of it. I only noticed it when I added windshield washer fluid and saw my overflow tank was dry.

Does any pulley make rattling sounds or does your drive belt squeak? Any whining sounds when you rev the engine?

Diagnose it using the steps in this video - 

https://youtu.be/evpaTW2WJ5Y

A failing head gasket can force air into the radiator and it will cause bubbles in the overflow tank. 

Also, a worn out radiator cap can cause overheating.

Have you checked out the radiator cap spring? They are designed to vent if the cooling system is going beyond the designed pressure of the circuit, like when the engine is overheating. Steam comes out of the cap when the relief valve opens, because the 16 PSI pressure raises the water's boiling point, and when you release pressure on superheated water, it shoots to steam. Driving on the highway raises the pressure in the system because the water pump's spinning at engine speed, and the engine is hot from driving at high speed. Your cap's spring could simply be weak and venting when it shouldn't. Replacing the radiator cap with a new one will replace the spring. Those are pretty cheap.     

Thanks for the help fellas but after all that trouble I have changed the cap, just to be sure. Also the mechanic told me the car doesn't have a blown gasket and the compression in the system is fine. Also there are no leaks and no oil to the water or water in the oil (and no "soapy" water or oil). 😕   This car is driving me maaad 😡   .

You may have a stuck thermostat. If your coolant is boiling over but the temp gauge says all is well, that is usually the culprit.

Also check your fans and make sure they are working correctly. 

Change the radiator and see what happens. I doubt that the catalyst does anything to do with the overheating issue you have. 

Fans and thermostat opens as they should. Also the radiator works as we took the car for a spin (with the mechanic) for half an hour (here in Greece it is very hot in the middle of the day) and it was just fine (we run the car at high rpm for half an hour to test it at the max). The fan opened as it should, and the thermostat also. Last time that i had the problem, we had also run the car and it was ok. But the problem appeared again after 200 km 🙁 . So the mechanic told me he had not see any car doing this stuff. Oh also the heater works (it is blowing dead hot inside when i open it). I don't know what to do, or where i should search... Found on the internet a guy that says it is the grounds of the car, but when i told this to the electrician he laughed at me.

Today, the temp gauge showed that my car is overheating 🙁 . After 3-4 seconds the temp went back to normal, only to go again to the red line (high temp) ❗ . And then it went back to normal 😡 . I didn't see the water boiling but i am almost sure the temp gauge showed right. I don't know what to do. Please, any help would be acceptable 🙂 . Thanks again anyone who sees it and many thanks to anyone that answers 😳 .

I’m sorry you’re having this trouble. how about the sending unit that goes to the temperature gauge? The last post you made makes it sound like the temperature is going way up and way down quickly. May be a faulty reading going to the gauge from the engine block. That would not explain boiling. Also, I know the thermostat has been mentioned. But sticky thermostats are funny. Sometimes they can stick open, where are you don’t notice a problem,   and sometimes they can stick closed. 

The engine doesn't make any unusual sounds, or squeaking noises, or rattling. It is very smooth. But the temp gauge after 4 hours of driving @ 90-100 km/h (5th gear @ 2000-2500 rpm) it showed again high temp just for 20-30 sec, and then the temp went back to normal. I done all the checks that you say in the video (thanks by the way) and checked everything else the all guys here say (again, thanks to everyone) but no luck. The engine loses no combustion, and it is tested. Changed thermostat and radiator cap. Then took the car for a spin with my mechanic. We drove it for an hour @ low, med and high revs and is running fine. He says if it was gasket it would show in the compression tests and the radiator is fine (after the second test). He told me again (after many times in his garage) that the fault is maybe electrical or electronic. But after 2 times in the electrician he told me that he can't find anything (hooked the car with obdii). So the car (i don't know why), randomly overheats, but just for some seconds. I don't think that it can overheat for some seconds, but it happens. Is it possible that the catalyst (it needs changing within the next month's), that makes all the problem? Or where to look for electric/electronic problem?? Already checked ECU and is fine.