Engine Stumbling

I have a 96 XJ Sport 4wd with 170,000 miles on the original 4.0 liter and automatic. After the vehicle has been driven for an hour to hour and a half, the engine starts to surge or stumble. I have replaced the crankshaft sensor, fuel filter, catalytic converter, coil, spark plugs, plug wires, rotor and cap. Also have replace the air intake temp sensor, coolant temp sensor and the idle air control valve. Does anyone have any idea what to do next? Could the O2 sensor be bad?

My 96 is an OBD I export model, I can turn the key on and off a few times and retrieve the codes from the computer. Have you checked the codes? An oxygen sensor code will turn on the check engine light.
My gut tells me it´s fuel related, fuel pressure test? I´ve heard of many 96´s with fuel pump problems, but don´t run out and buy a fuel pump.
I´ve also seen the same thing, in other manufactures cars, from a slight head gasket leak. Swapping exhaust gases into the cooling system and coolant into the cylinders, can show up at highway speeds and the only real indication, before the motor gets good and hot is a slight miss. Do you ever use/add any coolant?