I have been researching this forum for stumbling issue with my vet for weeks and I have read that the problem is everything from the fuel pump to the ecm here is what I have found out. If you have installed new ejectors, tune up, coil, EMC, TPS, ect., and your car still falls on its face the problem is in the tank. either the metal pickup tube has a hole in it or the rubber hose is loose. My problem after installing all the aforementioned parts I just didn't tighten the rubber hose clamp tight enough when I installed the new fuel pump. Trust me its cheaper and easy to inspect and test the fuel pump -vs- everything else that you might have been told to check regarding stumbling issue with half tank or quarter tank of gas.