Your difficulty indicates a problem with fuel flow. You'll need to check yoru fuel tank to make sure that the fuel pickup isn't in the front of the tank and becoming unported when you hold the model nose-up. That's the most likely difficulty. Another would be that there is a restriction in the fuel line or vent line that prevents proper fuel flow. Holding the model nose-up causes reduced fuel pressure due to gravity, and the the engine now leans and dies.

If the engine will run "all day" horizontally, then there's nothing wrong with the engine, itself. The problem is in the engine/airframe/fuel system combination.