Ford Repair: Car hesatates at cruising speeds, spark plug wires, gas mileage

Question
94 tbird 4.6 auto trans,car has only 60.000 miles and after cruising for about 15 minutes or so the car hesitates or stumble for a second and stops for awhile and does it again.sometimes it doe not do it at all.
i checked the plugs and they were all burning the same except the electrodes were white in which i believe that means its burning lean.I cleaned and checked the gaps and all the gaps were good.I changed the trans filter and replaced the trans oil and there was no differance.The trans fluid has no bubbles on the dip stick and there was no metal shaving on the magnet in the trans pan,It kinda seems like for a split second its not getting gas or spark or even a closed choke like in the old days.It does not seem like its trying to downshift,but I'm not sure.The trans always shifts good and overdrive seems to work like it should.If i step on the gas when it stumbles it takes off just like it should and i don't feel and slippage in the trans.The code scanner comes up with no codes and i even disconnected the battery for a few minutes to reset the computer.gas mileage is good,25 per gallon free way and 20 average.I would like to take to car on a 500 mile trip but i would like find out whats wrong before i do.
I'm just don't know if its a engine problem or a trans problem.It does not happen city driving.
Thank you for any advice
Gary

Answer
One weak spot in the 4.6 is the spark plug wires. Ford had updated these about 8 times when I left the dealership. Inspect the plug boots carefully for a white spot or a possible tiny hole where the spark is leaking. The other thing that comes to mind is the trans or torque converter, since you changed the fluid that should have made a difference if the problem was in that area.

If the plug wires have cylinder numbers on them, they are original and probably need to be replaced with a quality set.