The 98 explorer starts fine, most of the time.

Sound like you have a starter that starting to go out, the easy way to check it is when it only clicks have some one try to start it and you tap on the starter if it starts replace it, starter going bad.

I had the same problem with my 97 explorer, it probably has to do with your anti-theft alarm system, when it doesnt start just hit your unlock button on your keyless entry key, until the car starts. Usually hear a ticking if the car will not start, then once you hit the unlock button a few times it stops. Hopefully that helps.