I suspect the problem might be engine related. Dying in a dive is usually after a long dive. I takes a long time to use up the fuel in the line at idle. What is more typical is that as fuel is burnied in the line air is drawn in after the fuel. Then when the plane is at the bottem the line starts to pick up air behind this air and so there is a large bubble or a lot of medium bubbles. So then the engine leans out or dies on the upline.

Maybe a hotter glow plug would help?