Fifth gear engine miss in a 1990 Mustang GT

My 90 GT has a miss in fifth gear while driving on the highway. This is the only gear that I notice the miss in. If I am going 70mph and give the car steady throttle to increase my speed to 80-90 mph sometimes even though the car will continue to accelerate you will feel a miss and this also sometimes happens if you floor it while acelerating in fifth. If you downshift to fourth I do not notice it and I do not notice it while going through the gears around town even when running the car a little hard. I never noticed this miss before in the 20,000 miles that I have put on it. I think the distributor cap and rotor may be getting old but do you have any other suggestions.