I think the problem is the 2 psi pressure at the needle valve. With such a low pressure, the flow through the valve is dependent on a lot of factors other than the geometry of the flow passage. And, those factors are not stable, so the flow changes. I found I got much better results with a pressure around 20 - 30 psi at the needle valve, even though adjusting the valve is very difficult, with my cheap Milwaukee regulator, compared to at a lower pressure.