If this is just happening when on an incline, then it really does sound like water or trash in the fuel tank. Did the dealer try draining the fuel tank? What about the fuel filter/pressure regulator (one unit)? Does it just happen on an incline? What about when on a decline?

fyi ... water in gas station tanks is not uncommon ... especially when it is an older gas station that still has steel tanks, and especially when the water table in the area is high enough to reach the gas tanks.