Thank you, for all your answers
I've also changed the thermostat, which was unnecessary. Old one was good, temp was constantly at 100C (212F)
We've replaced blend doors. Passenger side door was broken when i bought the car, so replacing the heater core will not be a problem.
Coolant level is staying consistent. There is no blue smoke from exhaust, so I think head gasket is good.
I'll check the pollen filter. First need to find out what it is and where is it located.
I've used one can of stop leak. Guess I've used it the wrong way. Last time the heater worked was when I've put in the stop leak.
Every time, I back flush it with compressed air. Need to apply quite strong pressure. Last time I put apple vinegar in the heater core overnight. There was a lot of gunk in the heater core when I flushed it.
There is no oil in cooling water.
It is a very weird problem. I cannot figure out what is wrong. I will change the heater core and then we will see if that helped.
The car has 143 000 miles
Any other suggestions??