Wont Start

My 1999 Ford Explorer Sport 4.0 OHV engine has randomly not started for some time. The engine cranks at normal speed. Usually I would try several times or wait about 5 minutes and it would finally start. The frequency has increased. Today I would not start. I have disconnected the negative battery post to reset the computer. I listened at the gas cap filler and had someone turn the car on. I think I hear the fuel pump turn on (high pitch whine). I finally ran the battery down. Put car on charge. Went to parts store and bought starter fluid. I wanted to separate electrical from fuel. On return I removed the air inlet from the throttle housing. Squirted a little starter fluid. Car started and stalled. I replaced the inlet pipe and it started and ran. (May have been air sensor right after inlet box). What is possibly causing these starting issues. I bought a new fuel filter but the filter has been replaced before. The Fuel Reset is not lit and I manually pushed the fuel pump reset switch anyway with no effect.