How low was the gas in the tank? There could be a fuel pickup problem if this only happens when the tanks getting empty. Try it again with a full tank. Going up a hill puts more strain on the engine that may cause piston ring blowby if the rings are worn. Not usually a Volvo problem unless the engine has not been maintained properly with regular oil changes. What's your mileage? A compression test may show you something. All the readings should be within 10% or so.