If it works when you first bleed the system and then stops, it almost sounds like there is a head gasket issue allowing compressed air to enter the cooling system. Is the cooling fan turning on when it is at idle to lower the temperature?
As far as a test, here is a link that explains how to test for a bad head gasket. There are a few different things to read through, but I want you to pay special attention to the one discussing air bubbles in the coolant. It's approximately 2/3rds or the way down the page. If you notice bubbles, there is a chemical test that can be done to see if exhaust gases are entering the system. The link explains how that is done.
Let me know what you find or if you have other questions.
Thursday, July 1st, 2021 AT 1:07 PM