philip053,
Have you checked the tubing that goes from the fuel tank stopper to the clunk? If this hose has so much as a pin hole, it could cause the fuel to not be sucked through it especially when vertical. It may be submerged in the fuel when horizontal but when vertical it may surface out from the fuel and cause the engine to stop. It may not even be visible to the eye, ask me how I know this... I had a similar problem and it ended up to be a tiny hole in the clunk tubing. When you point the plane vertical, does the engine speed up just before shutting off? Hope this helps...