So to make things clear, it only does this when you go up hill and not anytime else?
If so, it sounds like something fuel related. Is it fuel injected or carbureted? I would check for fuel flow and to make sure there is enough fuel in the tank.
Seems like there was someone her that was having a problem going up hill but not downhill or on level ground and it turned out a wire shorting to ground.
Perhaps a wire worn through rubbing a moving part like the steering column or shaft on one of the pedals but only makes a solid connection to ground with the help of gravity and a hill.
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