2000 Buick Lesabre Hesitiates going up inclines

2000 Buick Lasabre, 98000 miles. When driving the freeway 65-70 mph, going up an incline the engine will occasionally hesitate, not stall, not backfire, but jerk like it's not getting enough gas. If I take my foot off the accelerator for a second and then re-apply it, the action will stop and I don't loose much speed. I've had the fuel filter changed (it was dirty). The mechanics are telling me it's the fuel pump. Could it be the O2 sensor because it starts fine & runs fine on flat surfaces and going downhill and not every incline causes this response. Could it be the fuel injectors? I'm also due for a tune-up @ 100,000 miles. I've owned the car for 7 years and keep up the maintenance as reccomended in the manual. I drive at night in the heat of Las Vegas and don't need to be stranded somewhere.