What would cause me to have vaper /steam in the radator

What is your question?

I can get it to bleed an run right for a couple days then i have steam vapers in my radiator. Changed thermostat flushed heater core checked for leaks cant find any.dont have a clue where to start next

What makes this problem better or worse?

When it get warm

How long has your 2001 Oldsmobile Intrigue had this problem?

Few weeks