Might just be that your fuel gauge is inaccurate. I know when mine has been at 1/4 tank it has sputtered on hills. I now acctually now go by the trip oddometer, whenever I get to the 450km mark I start looking for a place to tank up. I know at 500km it'll take like 50 litres or more in the 55 litre tank, but the gauge is just a little below the quarter. So try dumping a couple of gallons in. Then go to a gas station and fill it and see how much gas it'll take to see if you were just empty.
Alec
