Check the distributor for play. I had mine running, pulling a tear drop camper up to Camp Jeep in WI 2 yrs ago. It started to buck, hesistate on the highway at 70/3500, so I slowed it down to 65/3200 and it was fine. Got to a gas station. I wouldn't start after filling up. Distro rotor had lots of sway to it. New distributor and it started right up and hasn't been a problem since. Also check your ignition system - coil, wires, cap/rotor.