Overheating issues

[Reynold]

Hi all,
my car is sometimes quickly overheating when in stop and go traffic. The level of coolant in the radiator is always descending, while the level in the overflow has raised. Cold air is blown through the vents sometimes when idling too (that is when the temperature raises). The mechanics did a compression test, and the head gasket looks fine, so he changed the thermostat, which did not fix the issue. Air bubbles come out of the rad when idling. Any ideas ? My car is a 2004 Civic SE with 198xxx kms

 

[Reynold]

Top off the radiator with coolant, and then pressure test the cooling system. HFT sells a test kit.

https://m.harborfreight.com/radiator-pressure-tester-kit-63862.html

Mechanics did that, too... Everything looked fine. I am thinking about a bad radiator cap, but the bubbles ? Is it the coolant boiling ?

 

[Reynold]

Finally it was the rad cap which was making me lose coolant, but because I overheated after that, I blew the HG... Now selling the car because I have no money to repair it.