Help with starting problem on my 99 Jeep Grand Cherokee

i have a 99 jeep grand cherokee, it has started shutting down randomly and re starting immediately,it has worsend to not starting at all except once in a while and when it does it starts and dies within a few minutes. i replaced the crankshaft positioning sensor and fuel pump about 8 months ago. also, it acts like it wants to start, so a friend of mine put an air nozzle down into the fuel tank and blasted some air into the tank and it ran fine for a month, and now wont start unless air is pressurized into the tank. any suggestions would be much apprecfiated