Hi and thanks for using 2CarPros.com.
Since you have already replace the thermostats, bled the air from the system, and the cooling fans are working, I would recommend flushing the radiator. You mentioned using a stop leak product. Often times, they can actually melt down and plug the radiator, heater core.
Take a look through this link:
Next, since coolant is disappearing, either there is a leak or a bad head gasket. Here is a link that discusses the process of locating a leak:
Last, here is a test for checking for a head gasket issue:
Take a look through these things and let me know if you have other questions.
Tuesday, February 16th, 2021 AT 10:23 AM