Blockage or leak in fuel line? Choked filter? Maybe needs a service. Assuming that it hasn't done an enormous mileage and it's just plain worn out. Another simple thing could be....are you lugging any extra weight about in the boot that doesn't really need to be there? That in itself can make a difference.
my friend had a similar problem with a second hand car. After trying evrything he could think of a mechanic told him it was the fuel ejector that wasn't getting enough fuel through to power it uphill. This sounds simialr?????