The tank is "skinny" in the up hill/down hill direction. So that is not your problem. As Bill says, maybe the pump is weak. Maybe a filter is partially clogged, etc.
I'm trying to relate the 1/2 tank syndrome while knowing that the fuel level is not the problem. Only thing I can think of is: as the tank level goes down, it tries to draw a vacuum. If your gas cap is no longer venting, the tank will draw a vacuum against the flow/fuel pump and starve the carb. Notice any noise when taking cap off? Next time going up hill and it stalls. Take cap off and try it.


Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn