My Jeep keeps dieing. I need help

I need to figure out what is wrong with my Jeep. A mechanic suggested changing the spark plus and wires since he couldn't find anything with the diagnostic machine, which we did. We had already changed the fuel filter.

It has been cranking over but not starting, but sometimes after it has sat in the driveway or where ever it has died at the moment it will start several hours later. It has died for no reason on me while driving on the highway as if I had turned it off. There was no spitting or sputtering as if it was running out of gas. It died on my husband while he was sitting at a stop light and would not start again until about 4 hours later. I have been driving it and had it die on me, but would start back up after putting it into neutral and turning the key but it is not a common thing for it to start back up so quickly.

I read about a 1999 Jeep Grand Cherokee that was doing alot of the similar things, but I know car companies like to change things around with different years. I did not want to just assume all the suggestions given to her would also pertain to mine. If someone couple please help me out, I would very much appreciate it. The issues have my husband stumpt and I really need my Jeep so I can go to work. We can not continue to try and share a vehicle since he goes into work earlier and gets done before I do and our jobs are 30 minutes in the oposite directions. He tried to walk home day, that was a huge mistake.

Please help. Thank you.