Fuel pump or fuel pump filter

I have a 98 Plymouth Voyager SE that after parking on a slight incline with about a 1/4 tank of gas wouldn't start. After adding a gallon of fuel it started right up and ran ok. Another occasion of parking on an incline with almost a half of tank of gas the problem occured again. This time after adding a gallon of gas it did not start. A friend of mine took a wrench and hit the gas tank a couple of times and it started up and seemed to run ok. Now after not having driven it for a day, it won't start. Could this be the fuel pump or just the fuel pump filter?