Your truck has electronic ignition and what is called a vacuum advance.
Inside the distributor, there is a pick-up coil. This basically replaced the points, but that's not important here. The pick-up coil generates a signal each time a fin on the distributor shaft passes by it. 1 fin for each cylinder, blah, blah, blah. There are 2 wires going from the pick-up coil to deliver the pulse (signal) to the ignition module (the Japanese cars call them ignitors. The ignitor uses this pulse to tell the ignition coil to spark.
The pick-up coil is mounted on a plate inside the distributor. This plate can be rotated by the vacuum advance. This is used to change ignition timing slightly to make the engine start easier, make more power when needed, and give you some gas mileage.
Anyways, the plate has moved a whole bunch of times since 1986. The wires from the pick-up coil become hardened with age and heat. Eventually, these wires will develop a break inside the insulation, or right at the pick-up coil. When the plate is in a certain position, the wire is making enough contact to run normally. As you accelerate, the vacuum advance moves the plate until the wire loses connection. This will stop the spark. Sometimes, if the engine is turning fast enough, when the engine stumbles, the vacuum drops, the plate goes back to the original position, and we have spark again, then, the vacuum picks back up, the plate moves again, and on, and on, etc.
It usually feels like someone is turning a switch on and off.
The pick-up coils are not very expensive, and you do not need to remove the distributor to replace it. I would suggest having a qualified technician test it and replace it, or at least buy a Haynes or Clymer manual for your truck. They will show you step by step how to do it.
