A possible cause is moisture in the distributor as several have pointed out. I had a recent problem with my 1988 Chevy which had similar symptoms with idling etc. as you described. It turned out to be the Ignition Control Module connection inside the distributor. The module connectors outside the distributor are pretty waterproof, but the one inside is not waterproof at all. Mine had considerable rust on the two terminals when I disconnected it. It must have been giving a weak spark for some time. Don't just clean those two flat terminals, but also clean the spring connectors where they slide into. I'm not sure if your truck has the same setup, but I recommend anyone with the same module check that inside module connection. I had always overlooked that connection until I was forced to figure it out. Don't forget to use some dielectric grease on the connection too.
If this distributor there is a fin or fan shape on the bottom of the rotor. If yours has this, it means that there is a screen in the bottom. If plugged, it builds up with oxygen ozone and creates an acidic coating on the rotor terminals, cap terminals even the connector to the ignition module. Now if apart, shine a good flashlight all the way around the timer core on the shaft. You are looking for hairline fractures that will interfere in producing the 450 Mv required to trigger the ignition module off to collapse the coils magnetic field producing spark. I am happy to see Dielectric grease mentioned but as it should be used on every electrical terminal in the vehicle, under the ignition module, the grease used there was a white paste. This is thermal conductive heat transfer grease. Purchased at Radio Shack, electronic parts stores, Amazon it is different and a little more than Dielectric Grease. Inspect timer core for cracks, use heat transfer paste under ignition module and use compressed air to clean the vent at base of distributor. As far as Dielectric Grease, but it at the home improvement stores in the auto section where epoxy is hanging, it cheap. I use it on headlight connectors, PCM, stop lamp switch, behind tail lights so they don’t turn green and stick, but flashlight, beard trimmer, smoke detectors 9 volts, electric toothbrush, well, you get it. ASE Master Tech since 1978 Retired.