Maybe it's just the radiator cap that opens way too soon. If not... There's too much pressure on the coolant system. Probably head gasket failure.

Friend of mine had the same with his nsx. Leak down test was ok on all 6 cylinders. Still, headgasket was blown.

 

I did do a CO sniffer test and the results were good, so that would indicate the head gasket is ok.

Yesterday I pressurized the system with a tester unit to 2 bar. That's when I saw coolant coming out of the heating system line near the firewall.
That could be the leak. I fixed the line and tested the car again. But it's freezing here and I can't really push the car. But after an hour driving there wasn't any air in the system.

Fingers crossed this simple test found the issue.

 

I also tested the radiator cap and it's fine. To be sure I bought a new cap.

If there's still a leak after this then i'm pulling the head and getting a new headgasket.

 

My friends HG was only leaking in high rpm, with some real pressure on the system.

Be sure not just to replace the gasket, but also check if your head and deck are straight

 

Any word OP?