SHORT ANSWER: Disconnect the ground terminal from your battery vount to 30 and reconnect. This will sort of reboot the pcm. if your jeep starts up its all about the pcm or the ground cable. Check ground cable from battery to engine block and any other ground cables. you can buy a lifetime warranty pcm online for about 250 which is less than having a mechanic trouble shoot this mess. I am told the pcm internal power supply or capacitors fail over time and the unit becomes intermittent. I just ordered a new one but have yet to receive adn am running off teh reboot with a tightened ground cable which was slightly loose.
LONG ANSWER: I bought a 1994 Jeep Wrangler Sahara from a friend. Its in decent shape and my son and I are having a blast working on it and driving it. BUT when driving it would suddenly cut out. Fuel pump dead, coil dead etc. it would crank but not start, acting like it wasnt getting fuel. We rewired fuel pump changed coil etc. I read several threads and found a video describing a loose PCM that shook the plug out and caused same problem. Checked ours and bottom screw fell out and it was flapping against firewall. (The pcm is located behind the washer fluid bottle which is easily removed with one inboard phillips screw). it is a black box with three screws mounting it to the firewall. Also the screw holding the plug into the pcm had torn out and the plug had been glued in. I took it all apart and hit all the connections with chlorinated brake cleaner, reconnected and it is running fine now. I have read numerous threads describing the mysterious dieing and it coming back to the PCM and battery ground cable connections. Hope this helps, I was stumped for about a week.
Also installed a posilok device because the vacuum activiated 4wd is unreliable. I highly recommend this and there are several videos online showing the install.