1996 Chevy Suburban does not fire..has spark but will not start

[Dawn]

We keep about a half of a tank of gas in this vehicle but it is parked all the time. We would start every day because if we didn't it was either hard starting or wouldn't start at all. A couple weeks ago just stopped firing. It has a spark but won't start. We replaced the coil and still will not start. The sparkplugs are sparking. The fuel is pressyred up in the injection manifold. We can hear the pump coming on. Starter fluid does not start. Just keeps acting like it jumped timing but how can it jump ttiming if it is just sitting there.