Oh, THIS sounds VERY familiar! I had this very same problem, so let me share some possible causes:
Number one, and this was my case, bad ignition. In my case (YJ) the coil would get too hot, and then it would not fire well enough to ignite the mixture. Replacing the coil would fix the problem temporarily, but it would start again in a few days. The solution (so far; I have not driven the Jeep long enough to be 100% sure I fixed it) was to replace the entire ignition (found corrosion in the distributor cap and rotor, as well as inside the boots). The ignition (rotor, cap, wires, coil and plugs) is not so expensive to swap out, and, in the worse case, you will have done a tune-up if it was not the cause, as all these items (with maybe the exception of the coil) need to be replaced periodically anyway...
Another probable cause is a bad catalytic converter. If the cat is clogged, when the heat from the exhaust gets to a certain temperature, the cat will expand a little bit, and it will completely block the exhaust, which will cause the engine to stall.
The last thing that could be would be a bad fuel pump or a clogged fuel filter. In this sense, I would start by replacing the filter (again, could be part of a tune-up). To determine if the fuel pump is to blame, you would have to run the Jeep with a fuel pressure tester plugged in, as otherwise, it could show you have the right pressure at idle, but it could be failing under load...
Hope this helps!
