Can't figure this out

I've got an 89XJ 4X4 with a 95 4L in her. Sometimes she just won't start. Doesn't matter if the engine is hot or cold. She'll turn over but not start. I can hear the fuel pump working and it's not the battery or the cables. If I wait ten to fifteen minutes she'll usually start right up. I can go weeks without it happening them "Wham" she'll do it. I'll go out and do some running around and all will be alright then I'll jump in and she won't start.
I've taken her in for diagnostics and they say that everything shows good. The only time they'll be able to determine what is wrong is when it won't start but I don't want to pay someone my hard earned dollars to do something I can do. I just need to figure out what it is I need to do.
Could it be the computer? Has anyone else had this problem?

Did you retain the Renix head? How many miles on it? Does it use any oil?
Need a little more info Rabit.
Pull the air filter out and see if it will start then. Some older engines have "blowby" and will get enough oil on the filter to impede air flow thus you are missing one of the key things needed to have combustion....air.

jiggle the wires on your cps at the rear of the head, you may have an impending cps failure, it happened to me

Doesn't burn the oil, top end of motor was just rebuilt(Did it before and continues since then). Air filter is clean. The motor is good. I've whacked those wires around a few times, even taken them apart and blown them out. I'm pretty sure it still has it's renix head.

Only thing that has not been replaces or upgraded that could exhibit the symptoms you gave us is the CPS so if unplugging and re-plugging the connector does not solve the problem then it is replacement time I'm afraid to say. IMHO you should try that.

yep, and that cps is much easier than many folks have let on, you just need to have the correct set of tools for yourself, read up and you'll get the idea, there's a specific combination of Ujoints and extensions and such to get at the hardware just so...