I found this thread searching for why my Iwata Eclipse is bubbling in the paint cup.
My airbrush was fine. Then I replaced the needle and seat with the .5mm set and now I get bubbles in the cup. Brand new parts so I don't know how they would be clogged with paint, or damaged. I put them in and the problem occured right away.
The bubbling happens at a very specific time. Maybe this will help.
If I push the trigger for air, no bubbles.
If I am pushing the trigger and pull it back for full paint release, no bubbles.
The bubbles are worst right as you are pulling the trigger back, and when you are spraying with the trigger not pulled back very far.