Why does my 1999 jeep grand cherokee keep stopping without warning?

There are several things that can cause a stall. Most common is the Crankshaft Position Sensor. A failing Fuel Pump or Ignition Coil can do it as well. Is the "Check Engine" light on or does it come on before it stalls? Does it start up again immediately or does it have to sit for a while and cool down before you can start it again?

Thank you for answering. The engine lights come on once it stalls. Most times it starts right up, a couple of times it took about 30 minutes, and the other day it was out for over 3 hours (I had to have roadside assistance tow me home). I took it to the shop and they couldn't find anything to

...attribute it to as a result of their computerized diagnostic. What do you suggest I have a mechanic look for?

That's a tough one because the mechanic will need to make it stall and check for failures while it's happening. The crank sensor is the most common failure though.